A market economy operates in the real world because of the complex interactions of many people, all of whom have their own best interests at heart. It can be difficult simulating this in a roleplaying game world, especially if you don't have a degree in economic theory. Which is why game world economics tend to be simplistic and not particularly realistic.
But you know, that's okay, as long as none of your players have degrees in economic theory.
(Or if one of them does, and complains about it, you can get them to create rules for how the economy works.)
C-3PO: Behold the Jedi who revitates me!
C-3PO: You were about to destroy this source of power. You must rearn to value your resources for their abilities, not merely their meat.
R2-D2: Some of us don't even have meat.
[SFX]: < zang bedoop boop ding-spip >
C-3PO: And estabrish market value of potential services before you agree to trade, or you will get ripped off... again.
Logray: Behold the teachings of the Great Reader! Punitive, but legally binding!
Logray: You'll give us a seminar, Reader?
C-3PO: No. I've sold you a seminar.