Since trainers capture pocket monsters and make them fight to unconsciousness, I don't think the trade of these creatures puts them in any worse a position. Getting angry at the selling but not the capture is like allowing slavery but not the trade of slaves in the real world — there's just not much point in accepting one without the other.
I'd rather cut a Slowpoke's tail off myself than pay such extortionate fees. I'd kill it cleanly before doing anything to it... but, amittedly, more to save myself the trouble of it flailing around than out of moral or welfare concern.
I wouldn't buy monsters unless I had a history of them, health records and the like; one doesn't buy a car without also acquiring the legal documents, after all. My prerequisites granted and the monster shown to be in good health, I have no issue with the matter. There's not much of a case for me buying one though, Pokeballs are no doubt cheaper and a venture into the woods would do me good. I only require a craven Spearow companion anyway.
To answer your last question: Since I'd be a whaler living in Sootopolis, if I lived in the Pokemon world, it wouldn't be in my business to sell entire live monsters — I'd only be selling Wailord meat and whatnot, heh. If I found a Lapras, though, things could change; they've been poached since the first generation games, and given their rarity, could fetch a good price and allow me to take a break from my job for a while.
I'm not nearly so much evil as apathetic, honestly.
