Scotty bought a boat? But they don't use money?

I may be mistaken, but the whole "Human race no longer uses money" thing is never really explained in Star Trek. It's possible that Starfleet officers earn credit for their time working. Or maybe it's a mistake.

I think you can lay the inconsistency at the feet of Gene Roddenberry. By the time TNG rolled around, he has decreed that the Federation was without conflict as well as the need for money. (The latter was revealed in an AOL chat but Ron Moore.) Even prior to that, the use of currency was inconsistent during the TOS years.

It's well known That Gene and Nicholas had VERY different views on the Trek universe. Gene strongly disliked Meyer's militaristic approach. When TNG was originally conceived, Roddenberry insisted that conflict within the Federation was non-existent. It had become a utopia, so the use of currency would naturally have been unnecessary. The lack of conflict is often cited as why the writers struggled so much in the first couple seasons, and why the show became more interesting once Roddenberry's influence was gone.

by the time Trek VI was being made, Roddenberry was very ill and died nearly at the time the movie was released. Meyer's vision prevailed and I doubt he cared one bit as to how consistent his ideas were with the utopianism Roddenberry espoused. Humans need conflict, a protagonist and an antagonist, for stories to remain compelling.

If you want to pretend it makes sense, I'd suggest that internally to the Federation money is meaningless. Why would anyone need money when replicators can produce anything you can possibly want for free? However, the need to interact with other species would require the Federation maintain a supply of foreign currency. Each member of the Federation might receive an allotment of currency to use when the need to trade with other species arises. For instance, how would anyone aboard have bought a drink at Quark's when you can be 100% certain he takes only money? If every one of your needs is met, a small spend from the Federation would be so you need in the rare instance you want something the Federation can't supply.

Memory Alpha has a page that partially covers the controversy.

http://memory-alpha.wikia.com/wiki/Money
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In space, no one can hear you scream. On IMDB, we can hear you but we just don't give a crap.

I think you can lay the inconsistency at the feet of Gene Roddenberry. By the time TNG rolled around, he has decreed that the Federation was without conflict as well as the need for money. (The latter was revealed in an AOL chat but Ron Moore.) Even prior to that, the use of currency was inconsistent during the TOS years.

It's well known That Gene and Nicholas had VERY different views on the Trek universe. Gene strongly disliked Meyer's militaristic approach. When TNG was originally conceived, Roddenberry insisted that conflict within the Federation was non-existent. It had become a utopia, so the use of currency would naturally have been unnecessary. The lack of conflict is often cited as why the writers struggled so much in the first couple seasons, and why the show became more interesting once Roddenberry's influence was gone.

by the time Trek VI was being made, Roddenberry was very ill and died nearly at the time the movie was released. Meyer's vision prevailed and I doubt he cared one bit as to how consistent his ideas were with the utopianism Roddenberry espoused. Humans need conflict, a protagonist and an antagonist, for stories to remain compelling.

If you want to pretend it makes sense, I'd suggest that internally to the Federation money is meaningless. Why would anyone need money when replicators can produce anything you can possibly want for free? However, the need to interact with other species would require the Federation maintain a supply of foreign currency. Each member of the Federation might receive an allotment of currency to use when the need to trade with other species arises. For instance, how would anyone aboard have bought a drink at Quark's when you can be 100% certain he takes only money? If every one of your needs is met, a small spend from the Federation would be so you need in the rare instance you want something the Federation can't supply.

Memory Alpha has a page that partially covers the controversy.

http://memory-alpha.wikia.com/wiki/Money
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In space, no one can hear you scream. On IMDB, we can hear you but we just don't give a crap.

I thought the currency at the time of TOS was Federation Credits? I mean we we saw bars in space stations where a thirsty ham couldn't get a drink for money or in trade.

As for Starfleet personnel, I imagine they had some kind of replicator usage allowance on board their ship. Sure, the replicators could provide them with all the food they needed and possibly clothing, but would it let them replicate something like hundreds of designer knockoffs to sell on non-utopian planets?

Some of the other replies are a bit more insightful but Trek seemed to flipflop on this issue often. In ST:IV Kirk and crew talk about money like it's some kind of foreign concept but in ST:III Bones is caught trying to buy passage to Genesis. Now we see Scotty buying a boat.

I'm not going to nitpick about it but it is one of the more valid nitpicks about the Trek universe.

Humanity went back to a system of bartering with seashells and beads.