Interstellar commerce is a tricky issue; some highly advanced civilizations posess the ability to replicate matter, so precious metals are essentially worthless when it comes to using them as currency (although as symbolic currency they might still be a viable, i.e. like the United States dollar bill).  So, what do people use for money?  What is truly valuable?

Bear in mind that various metals and minerals are still valuable, but the cost of transporting tons of iron ore or the like is cost prohibative due the cost of maintaining ships large enough to transport such materials in bulk.  The same goes for transporting food; "farming planets" are usually worlds that produce crops of rare cash foodstuffs rather than staple grains.  An example would be a world that creates a rare alchoholic beverage from a local fruit.

CREDIT CHIPS: About the size of Earth poker chips, Confederacy credit chips (or creds) are actually advanced pieces of technology containing microchips that denote their value when scanned by monetary devices.  They are notoriously difficult to tamper with and doing so for the purpose of monetary gain is a serious offense.  Confed creds come in three colors: white denote one cred, silver ten, and black are a hundred.

FISSIONABLE MATERIAL: A great many civilizations employ radioactive elements as power sources and such refined fissionable material is always in demand.

RARE PERISHABLES: Civilizations are always looking for the new, the unusual, even the weird.  Odd animals, rare herbs and spices, and various foods can all net a high value in certain markets.  But both the buyer and seller must beware that the perishables they are dealing in are not poisonous, or in some way can damage the eco system of a world.  Many races still recall the insertion of the adorable Whindon Whiskets to the planet of Bodo.  The higher oxygen levels caused the adorable Whiskets to breed like mad and within several generations an ecological catastrophy was witnessed.  The term "neck high in whiskets" is still used as a term denoting impending disaster.

ART: While diamonds might be pretty common in nature, it is not always the case with fine jewelry.  The same goes for paintings, musical recordings, and sculpture.  In the right markets such work can net high prices, while to other eyes (or alternative sensing apparatus) these things may seem to be nothing more than primitive junk.