Obviously, there are people more qualified to do certain things in the Star Trek universe than other people. After all, you can't just wander in the door and become a starship captain. So already, you have some unavoidable stratification by ability.

In any human society, some form of compensation for skills and labor will be required. In an advanced technological civilization, the actual work may be minimal compared to the reward. The average member of a 21st century Western society, even one who does blue-collar labor, still lives better than kings in medieval times.

In the Star Trek world, it's possible that even a small level of work or trade in the more advanced worlds could give one the current equivalent of great wealth. Here's an example over just a few decades, although I have some quibbles over a couple of his examples.

Obviously, once you get out on the edges of the Federation, economics becomes a little more 21st century, with credits, trade items, and gold-plated latinum figuring as currency (according to the canons, latinum apparently distinguishes itself as a rare material because it is impossible to replicate for some reason).

This speculation is all for fun, of course. I doubt the writers ever gave the economics of that world much thought (in fact, economics probably made them uncomfortable), and pulled in elements required to make plot points at any given time.

Other SF series like Babylon 5, Firefly, and others seem to have a more conventional idea of trade and economics. Whether this is a serious prediction of the future, or more likely, a simple attempt to put viewers on familiar ground, I don't know.