Too bad trek simply didn't go further to explain how humans don't use money or that there might be some type of currency exchange.
They just stated it, and left it up to the imagination.
One strange problem that comes out of all this if you think about it, is that under this idea, humans are under a type of financial lockdown--
If they leave earth, they will have no money or currency to do anything.
The state may have credits to do things off planet, but the average earth citizen wouldn't be able to buy food, clothes repair parts etc, because earth doesn't deal in money.