Quark has to pay for the space he occupies - he rents it from Deep Space Nine, which presumably at this point means the Federation. Those Federation credits he gets paid in can be used to pay for his rent, and other Starfleet/Federation equipment.
His profit comes from the other customers who pay in other currencies, including but not limited to, latinum.
Why does he have to pay for the space? The Federation is post-scarcity. Surely space is free. If space on DS9 is scarce, then how is it allocated? Fiat, apparently.
Star Trek doesn't bear up to any economic analysis. It's not even worth trying.
No, its not. Its just phenomenally wealthy compared to any place in the real world today.
If the Federation was post-scarcity, then many of the plot lines in the various shows wouldn't happen, and the Federation, notably, wouldn't be getting into conflicts with neighbors for scarce resources like territory, etc. (And if it did because the other party started it, it would wave its hand to generate an arbitrarily-capable defensive force, win the conflict, and be done immediately -- which obviously isn't what actually happens in any of the series in that situation, which seems to recur fairly regularly.)
What you're pointing out is that Star Trek makes no sense. The original Star Trek technical manual stated that they had what Neal Stephenson would later call "matter compilers" and used them for everything (the same technology that allows transporters to work creates food and clothing and recycles waste).
And Star Trek only gets more contradictory baggage as it goes on (in one episode of TOS they essentially resurrect the bridge crew using transporters; so I guess they could replace all the dead security officers too). It's explicitly stated, many times, that there's no money. But none of the implications of this are followed through.
And this nonsense suffuses the entire show, not just the economics. The new J.J. Abrams movies manage to be even worse, but at least they're fast-paced and the actors are trim. (Why does the Federation need to make deals with Khan when Spock just gave them interstellar transporters that can penetrate shields and target starships moving at warp?)
About the only thing that makes sense in Star Trek is that their security teams are utterly incompetent. In a world with infinite wealth and resurrection machines, there's little incentive to stay in shape and learn difficult (and painful) combat skills.
Perhaps the Federation actually is the Culture; perhaps the Minds of the Federation are staying hidden, and purposely confusing all these issues so that people have the impression that they have something to do.
Dave Barry once commented that the holodeck would be mankind's last invention and that transporters would make everyone morbidly obese. ("Beam me some nachos with queso.")
> Why does he have to pay for the space? The Federation is post-scarcity.
Deep Space Nine is not Federation property, however - while it is only administrated by the Federation, it remains the property of the Bajorans. Presumably, payment for use of space is part of the arrangement with the Bajoran government.
> Star Trek doesn't bear up to any economic analysis. It's not even worth trying.
Yes but you get higher and higher scores in Tetris.
Presumably the post scarcity Federation could simply build its own arbitrarily large and luxurious DS9 next door to it. In any event, space isn't likely to "cost" anything.
Everything about DS9 is different because it is on the far frontier outskirts of the Federation (technically outside,) where there is regular interaction with non-Federation peoples. The station was owned by the planet Bajor, which was a candidate to the federation, not a member. Bajor is not a rich, self-sufficient planet, it was devastated by war and occupation prior to the start of the series.
His profit comes from the other customers who pay in other currencies, including but not limited to, latinum.