Star Trek has always been terribly, terribly schizophrenic about this sort of thing. Diane Duane's novel Spock's World has perfectly good references to using money (Kirk pays his tab at the tavern and inn he was staying at when the book starts, McCoy remarks on how he signed up for a language course 'on his own dime').
IMO, it got worse in TNG when the writers realized a post-scarcity economy would look very, very different. They really should've spent more time fiddling with that. There's an amusing EU novel where the businessman the Enterprise crew thawed out from cryostorage makes a return; evidently the Federation hired him on because they needed expert advice in dealing in capitalist societies, particularly the Ferengi.