While Federation Credits are money, I was under the impression they were only really used when you want to buy stuff outside of the Federation from sellers who don’t share the same socialist society that the Federation has. On a Federation world I believe they wouldn’t have much value.

DS9 was not a Federation station, and the Bajorans were clearly ok with a capitalist presence. Giving Starfleet personnel a stipend seems pretty unavoidable if they want their officers to be able to partake in practically anything on DS9.

But that gives them inherent value and would end up being traded internally. And then people would buy up stuff from outside the Federation and charge people in the Federation for those things in credits so that those people don’t have to travel off-planet to get those desirable things.

And as I said, they gave thousands of credits to the Barzans, so credits are obviously worth something when exchanged back to the Federation too.

On top of that, in TOS, there is a scene where someone wagers with credits (conceptually, but it basically sounded like a thing). To add to that, credits were being used on Space Station K-7, a Federation space station, or Uhura would not have been able to purchase a Tribble and Cyrano Jones wouldn’t have been there selling them.

I’m afraid we will have to accept that Federation economics makes absolutely no sense whatsoever.