Make it so

[Colonel Flagg]

I think the dealio was that on the starship, everything was tied to your service in StarFleet. You could get whatever personal items you wanted, provided you were in good standing, and the items were not unreasonable (No Bentley's in Space!).

When you visited a starport not in Starfleet, or with non-Starfleet personnel (like Quark or Garak) running businesses, there was always an exchange of latinum - either strips or bars - for the service requested. It may not be universal, but it was used as currency, especially when dealing with the Ferengi.