I was looking for one that focused on it but must've missed it.There are whole threads on this (right next to "Is Starfleet a military") but the entire plots of Mudd's Women, The Devil in the Dark, and much of The Trouble with Tribbles. I might chuck Requiem for Methuselah in there as well. If you want to get into TAS then I give you Rich Dude Carter Winston.
I guess I should've also said, actual tangible currency exchange in Federation/SF by federation citizens to other federation citizens doing legal commerce.
I think any illegal transactions and those with species outside the Federation can reasonably use some currency... in fact let's not even say all the Federation, but Earth/human Federation entities. Then Winston and Paxton (ENT:Terra Prime) are the standouts in my mind, and Winston gets a pass as a "space trader," very likely dealing with non-Federation (or non-Earth Federation) species.