There is a long history of SF novels about interstellar free traders eking out a marginal existence on the fringes of the huge trader corporations, from Andre Norton's Solar Queen novels to the Space Angel series by John Maddox Roberts. Go to The Tough Guide to the Known Galaxy and read the entries "ECONOMY", "FREE TRADERS", "PIRACY", "REPLICATOR", "TRADE" and "TRADE FEDERATION".
Sometimes the traders live in large "clan-ships", developing a "trader culture." Each ship is a world, carrying the entire clan. Novels include CITIZEN OF THE GALAXY by Robert Heinlein, STAR WAYS aka THE PEREGRINE by Poul Anderson, the Cities in Flight novels of James Blish, MERCHANTER'S LUCK and FINITY'S END by C. J. Cherryh, RITE OF PASSAGE by Alexi Panshin, A DEEPNESS IN THE SKY by Vernor Vinge.
Naturally, to make interstellar trade work, you need the cost of interstellar transport to be incredibly low, or the value of the trade item to be incredibly high. Or both.
Raw minerals probably are not valuable enough, it will probably be cheaper to synthesize rare elements instead of shipping them in. As for manufactured goods, why not just send the blueprints by radio or by your Dirac Poweredtm FTL Ansible communicator? In a future where everybody has 3D printers and rapid prototyping machines, the economy would be based upon trading intellectual property.
Since there does not seem to be any real-world trade item worth interstellar trade (unless it is cheaper to ship from another star than it is from another city), you will probably be forced to invent some species of MacGuffinite.
In Larry Niven's PROTECTOR, asteroid miners prospect for magnetic monopoles (which are great for constructing compact motors and generators). Dr. Robert Forward proposed prospecting for Hawking black holes. In the old SPI game StarForce, the only valuable commodity is "telesthetic" women, who are the sine qua non of FTL travel, and who cannot be mass produced by genetic engineering. In Vernor Vinge's A FIRE UPON THE DEEP, some of the main characters are traders contracted to transport part of a huge one-time pad for secure cryptographic transmissions (such a pad cannot be transmitted without compromising security). Sometimes humans and aliens discover that one man's trash is another man's treasure. And in Charles Stross' IRON SUNRISE, the most valuable things are packages of entangled quantum dots, used for FTL communication via Bell's Inequality (with the fascinating twist that the dots must be transported slower than light or they are ruined. They are shipped by Starwisp).
The main mechanism for trade is what is called "Arbitrage", the practice of taking advantage of a price difference between two or more markets. In this context it boils down to "buy cheap and sell dear", that is, purchase goods that are cheap at Planet A, then transport and sell them at Planet B where the goods are expensive. The money you make selling at Planet B, minus how much you spent purchasing at Planet A yields your gross profit. Subtract from that your transport expenses and other expenses and you'll find your net profit (if any).
There is also the problem of price convergence. The profit is from the price difference between the two markets. The difference tends to shrink over time, which eliminates the profit. Sometimes the market at your destination becomes saturated (as the manufacturers of Beanie Babies found out), sometimes the supply at the origin dries up (like petroleum).
Traditionally, the unit of currency in science fictional futures is called the "credit". This is the futuristic equivalent of a dollar, Euro, or whatever. Using metric, one megacredit is a cool million credits. In some science fiction, physical money is illegal since the Police State cannot trace such transactions. Nowadays we have Bitcoin (BTC), symbolized by , Ƀ, or ฿. You can find an amusing list of the names of various fictional currencies here
- In SPI's RPG Universe and Star Trader, the unit of currency was the "Tran" or "transaction", where 1 Tran was equal to about $500.
- SPI's Star Force had "LaborCredits".
- In Philip E. High's The Prodigal Sun money was literally hours of work.
- In James Blish's Cities in Flight the OC Dollar was based on the Germanium Standard, because of its vital use in transistors and computer chips. This worked fine, until some joker figured out how to synthesise germanium and thus destroyed the economy of the entire galaxy.
- In George O, Smith's "Pandora's Millions", the invention of a replicator crashed the economy of the solar system. Replicators mean there are no longer any rare metals to base your money on, and all material goods become basically free. The only thing of value are personal services (such as those of a surgeon or doctor). The only thing that prevents utter disaster is a synthetic element that cannot be replicated (because replication causes it to explode). The element allows one to create cheques, legal tender, and other critical items that cannot be counterfeited by a replicator.
- In the Star Trek universe, the Federation is a post-money society that uses replicators. The Ferengi use "gold-pressed Latinum" as the basis of their currency, since Latinum is the one element a replicator cannot create.
- EVE Online has a little more complex a take on things. The currency, known as ISK (Inter-Stellar Kredits), is not so much a global currency as it is a global exchange currency. Planetary economies and sometimes individual planetary nations almost all have their own currencies, ISK was merely setup as an exchange medium to manage the obscene amounts of money being used at the interstellar level
- In the Micronauts series of comic books, the evil Baron Karza has a monopoly on lifespan prolongation technology (the "Body Banks"). He issues his own currency called "Life Credits", with which a person can purchase extended lifespan. The underclass waste all their credits in gambling institutions, and can sell personal organs and other body parts in exchange for more life credits. The aristocracy is firmly under control of Karza, since they know he literally has the power of life or death over them.
- In the Demon Princes pentology by Jack Vance the currency 'SVU' or Standard Value Unit was a printed note equal in value to one hour of common labor. A device called a "fake meter" is used to detect counterfeits. In the second novel the protagonist discover how to fool the fake meter, and hilarity ensues.
- In The Great Explosion by Eric Frank Russell, the planet K22g is a post-money utopian society, but they still have a medium of exchange. They use favor-exchange based on "obs" (obligations). This might explain the value of the poker chips you see in all those Star Trek poker games.
- In John Morressy's Del Whitby series, the unit of currency was the cash-cube. These were cubical coins of precious metal.
- In Diane Duane's My Enemy, My Ally, the Romulan's currency is in the form of chains of precious metal.
- In John Brunner's Intersellar Empire series, the currency is in the form of rings of preciouis metal.
- In Frank Herbert's novel DUNE, the Fremen's currency is based on liters of water, symbolized by metal rings. They tie the rings in strips of cloth so as to not make noise when they are sneaking up on an enemy.
- In the Battletech universe, a common unit of currency was the C-Bill, redeemable for a certain amount of data transmission on Comstar's FTL communications network.
- In the simulation game High Frontier, the unit of currency is "the most valuable thing in the universe", namely water. Water can be used for reaction mass, as a source of hydrogen and oxygen, radiation shielding, and a host of other uses. The unit is a 40 metric ton tank.
- In Kim Stanley Robinson's Red Mars Trilogy, calories of heat were used as the basis of the Martian economy.
- I am somewhat dubious about the Quasi Universal Intergalactic Denomination. Apparently it is intended to be safe in the space environment and will survive the space environment. This means it is constructed out of a space-qualified polymer, emit no toxic fumes, has no sharp edged, be resistant to high temperatures, and not use a magnetic strip like a credit card since cosmic radiation will render them inoperative.
- Of course nowadays most people use credit cards and PayPal.
Note that while Planet A and Planet B may both internally use a barter system instead of money, they might use arbitrary money (a nonstandard medium of exchange) when trading between each other. Using a medium of exchange avoids the problem of ensuring there is a double coincidence of wants, which is a problem inherent in the barter system.
Example: Floyd the guitar player lives in a culture that uses barter. At his job at the local bar, he is paid with booze and food. Unfortunately Floyd's landlord will not accept booze and food for the rent. In this case there is no coincidence of wants: Floyd wants his apartment for the month but the landlord does NOT want booze and food. However, in that happy double coincidence of wants, if Floyd's landlord happened to be throwing a party and wanted some guitar music, and Floyd wanted his apartment for the month, the barter system works.
The point is that a double coincidence of wants does not happen very often, which makes the barter system rather awkward and impractical. If however you create some arbitrary money-like medium of exchange, you can avoid the whole mess. It also avoids the problem of when one of the things you are bartering has a shelf-life or is seasonal.
Sometimes you will see traders using Trade Tokens. These are basically money that is not issued by a government, but instead by a private company, group, association or individual. From the 17th to the early 19th century these were used by merchants because the local government was not up to the task of issuing enough coins to allow business to operate. Nowadays you generally see them in the form of casino chips, in video arcades, and car washes. But a hypothetical interstellar trading company might issue their own trade tokens if there were no local government in the trade area, or at least one single government recognized by all the trade planets.
And scrip is an even more localized form of trade tokens. You often see this in old time mining or logging camps. The employees were not paid in money, but instead in company scrip. The company scrip could only be spent in the company store. Due to this Truck system, the employees more often than not wound up owing their soul to the company store.
Historically coins were disc shaped because that's what you get when you roll out a measured ball of precious metal and flatten it with a stamp bearing the King's face. They are also convenient to carry, especially if they have a hole in the middle for stringing. You should mill the edges to prevent crooks from engaging in the crime of coin clipping. Though nowadays most US coins are composed of such worthless metal that the milling is purely decorative or as a aid to the visually handicapped.
In medieval times there were so many currencies that merchants had to carry coin pan balances in order to determine the worth of a given coin.
Unless all the planets you trade with are members of the same interstellar govenrment, or there exists some sort of interstellar money-changing organization, the money used on one planet is worthless on another planet.
If you have a group of planets that share a common currency, for the planets sake it is vitally important that they share a common fiscal policy. The ongoing Eurozone crisis has been made much worse by the fact that while the Eurozone has monetary union (i.e., one currency, the Euro) it does not have fiscal union (e.g., different tax and public pension rules). This ties the hands of European leaders, making the crisis almost impossible to solve. When the Eurozone was proposed, the various nations were persuaded to surrender their currency, but reluctant to surrender control of their fiscal policy (give up their national sovereignty? Never!). The proponents figured to get around the problem by doing the Eurozone union in two stages, which in retrospect was an insanely bad decision.
Back in medieval times, merchant voyage durations were measured in years and long distance communication was non-existent. The same may hold true with hypothetical interstellar traders. In order to cope with the problems, medieval merchants invented Letters of Credit and Bills of Lading. Here is a simple explanation (with diagrams) of how they worked.
Medieval merchants had other innovations that might be useful in an interstellar trading future.
The roads were bad and in poor repair. Ocean routes were treacherous. Brigands and pirates lurked in parts of the trade route far from any help. Distant nations treated merchants with disdain at best and as rich people to rob at worst. And every single landowner along the trade route felt that they had a right to extort whatever tax they could get out of the trade caravan.
To fix these problems the medieval merchants found effective solutions, the most effective being the concept of a Merchant Guild. These were association of of traders. Guilds could invest the member's fees in such things as improving road conditions and suppressing pirates and brigands. Lighthouses were erected at dangerous points, to prevent merchant shipwrecks. The guild would negotiate treaties of commerce with foreign nations, protecting the liberty and security of guild members (sometimes the guild could even get an agreement for foreign troops to travel with a trade caravan). And while a single trader could not do much about landowner's imposed taxes, a huge guild could negotiate from a position of power. Negotiations with a landowner would result in a Merchant Guild charter, where guild members would pay a fixed sum or an annual payment for right of passage.
You can see how these concepts can be re-used in an interstellar trading future, the situations are much the same.
The flip-side of course is that the guild members have to pay their dues to the guild, and obey all the guild regulations. Members cannot engage in any type of trade forbidden by the Guild charter, fines were imposed on members who broke the rules, and guild members had to aid and support fellow guild members in times of trouble. If a guild member was killed, the guild would care for any orphans thus tragically created. Guilds also supplied health insurance, funeral expenses, and doweries for girls who could not afford them.
Naturally the guilds became quite powerful. Independent traders would find it difficult to compete. In a village, local craftsmen also found it difficult to compete with the Merchant guilds, which lead to the rise of Craft guilds in self-defense. Eventually the merchant guild members delegated all the actual traveling and trading jobs in their profession to employees, and instead sat comfortably at home while their factors did all the hard work.
Also interesting is how the rise of the 17th century Dutch seaborne empire was due in part to their superior utilization of wind energy in the form of their breakthrough cargo transport, the Fluyt ship. Unlike other cargo ships of the time, the Fluyt was not designed to be easily converted into a warship. It was pure merchant vessel. This means it was cheaper to build, carried twice the cargo, and needed a smaller crew. Specialized shipyards optimised for Fluyt production brought the construction price down to a mere 50% of a cost of a conventional ship. It could also operate in much shallower water than a conventional ship, allowing it to get cargo in and out of ports other ships could not reach. By using a Fluyt, cargo transport costs were only 70% to 50% of the transport cost with a conventional ship. The only trade route Fluyts could not be used on were long haul voyages to the East Indies and the New World, because Fluyts were unarmed.
If you are a science fiction writer or game creator, these ideas should start the wheels turning in your mind. It may be instructive to read a couple of history textbooks on the topic of Merchant Guilds, and look over the Nicholas van Rijn stories of Poul Anderson.
Space Pirates is a science fiction troupe that just won't go away. The image of pirate freebooters on the high seas is just too romantic for words, science fiction writers just can't resist. Alas, in a scientifically accurate world, they are more or less impossible, much like space fighters and for similar reasons. There ain't no stealth in space, so it is practically impossible for a fat space galleon to be surprised in mid trip by a sinister space corsair flying the Jolly Roger. Or a rude surprise for a space merchant ship whose trajectory passes too near the Somali Asteroids for that matter. It would be several orders of magnitude easier for the "piracy" to take the form of grand theft from the merchant's warehouses on the ground.
But of course if one is creating a fictional universe with faster-than-light starships, the author can tweak the properties of the FTL drive in order to allow piracy. As a matter of fact, many tweaks that will allow interstellar combat could also allow interstellar piracy.
As a general rule, merchant ships cannot be armed, armored, and combat crewed enough to fight off a pirate attack, not without increasing the amortized and operating cost and reducing the cargo capacity to the point where the ship cannot turn a profit. A "Q-ship" is a warship disguised as a merchant vessel, intended to fatally surprise hostile convoy raiders. They would also work against piracy.
Another problem to be addressed if you want piracy to be viable is infrastructure. Captain Jack Sparrow's ship needed no fuel, only the winds. The crew can repair much of the ship if they can find an island that has trees. And there is no shortage of places that will accept gold coins and jewels. Now a pirate starship might be able to squeak by if they can use water or hydrogen for fuel, but it will be a real problem if their ships require antimatter or highly refined plutonium. Repairing ones ship is job for a shipyard, not a random asteroid with the crew frantically looking for nuggets of titanium. And fencing high tech computer chips will be a challenge. In James H. Schmitz The Witches of Karres there are outlaw planets that handle these matters.