On Neopets (remember Neopets?) the trading of pets has to be conducted entirely through 1-to-1 pet swaps (unless you go to sketchy sites where you can buy painted Neopets for in-game currency or RL money), and I have to say that it is a giant pain in the ass, so I hope that pokemon breeders have the option of cash or goods along with barter.
Pokemon breeders would probably have a much more important role in a "real" pokemon world-- wild pokemon are dangerous and aggressive, such that you're barred from leaving your walled town before you have your own pokemon to defend you, so most regular people would get their first pokemon from a breeder who's socialized it and prepared it to be a trainer's pokemon-- or judged that it would make a better pet, or contest pokemon. And so you'd likely pay the breeder for that pokemon, as compensation for their time and energy in pre-training your new pokemon, but also toward whatever traveling and recruiting they'd have to do to find compatible pokemon for their breeding operation.
In the games, outside of the game corners in Kanto/Johto, there's merely the subtext of being able to get pokemon for money-- the rich kid/rich adult trainers you encounter often have unusual pokemon for the area (well, some of them just have a Zigzagoon or whatever), which seems to imply that they have the money for pokemon that the other local trainers don't, whether that means that they straight up bought a pokemon, or more subtly, their money gave them greater opportunity to travel and encounter different pokemon.
That said, the TR game corner always felt sketchy even outside of the fact that it was run by TR-- some of the pokemon sold there (dratini, pinsir/scyther, nidorin@) felt like poached (or resold) Safari Zone pokemon. :B It's easy to imagine those pokemon not getting a lot of care, too, like the guy just hands you a pokeball, and when you release the pokemon as far as it knows it's still in the Safari Zone, doesn't know who you are, and flips out.
There's an important subtext to catching a wild pokemon, actually-- your pokemon weakens the wild pokemon and you catch it, and your pokemon is loyal to you, so that wild pokemon looks up the chain and goes, "OK, charmander is stronger than me, and charmander listens to them, so I better listen to them". But when you get traded a pokemon (or, technically, given one-- but the games always give those gift pokemon your ID), that pokemon is like "who's this asshole?"
Anyway, tl;dr, buying pokemon is likely, but you better interact with and come to an understanding with that pokemon before you buy it; otherwise it's a risky proposition that doesn't establish the delicate social hierarchy upon which 10-year-olds training giant monsters rests.
Caveat emptor.
The treatment of Pokémon appears to vary a lot from 'mere animals' to 'human-level sentient beings' - and they are incredibly diverse creatures in terms of intelligence and sentience, after all - so really, the ethics behind selling them would probably be at least on the same level of complexity as the ethics behind capturing/training/battling them to begin with - pretty damn ambiguous.
This, the smarter you make pokemon the more dubiously ethical a lot of the core mechanics in the series become, including battling and breeding mechanics. (How we treat animals in our own world is a really fraught and complicated subject.) But if you go down that route, the exchange of pokemon for money pales in comparison to the greater issues of trapping and coercing sentient organisms, so.