I always thought the idea of no money was an altruistic Roddenberry-ism that is just not realistic. Holdfast explained it very eloquently, but there simply has to be some sort of value placed on the exchanges of resources and services. I think DS9 did a better job of showing the reality of currency.
I remember watching the episode where we see Sisko working in his fathers restaurant on earth. If there is no money then essentially the food is free, right? What's there to stop someone from just glutting themselves on everything with no restraint? Humans have been greedy and self serving since the dawn of man, is all that supposed to magically go away in just a few hundred years?
There is no problem with that whatsover. Moneless economy doesn't mean that if a citizen hides money in his house, police come, beat the living daylights out of him, then take him to gulag. Private people would stil have forms of wealth, it just wouldn't be dollars, because dollars are just paper.
Private transaction would stil occur no doubt, but major, official, transactions (and honestly, the ones that really matter) between state and citizens would have no money. If you want to buy a painting from a collector, you'd have to caugh up whatever he wants, whether it's dylithium, gold or something else. If you don't have what he wants, you'd have to barter like Jake did for that baseball card.
However, if Starfleet wants to build Enterprise, they wouldn't pay money for it. There would be some sort of reward for few
(since most of it would be automated) people that work on it, but they wouldn't be payed in currency. For example, there must be a reason why Kirk has a nice apartment in SF with a view, while some bum who spends his days in a bar doesn't.
Humans are stil gready, they're just greedy for knowledge, creation, science. In 24th century, they don't care about having the latest Iphone.