I see no wrong in it, so long as the Pokemon for sale were fairly treated, and were legally obtained and whatnot. If we are allowed to catch and trade them, then I see nothing wrong about what is basically trading, but instead of another Pokemon you get money. And, depending on their medical records, general health, and whether or not I felt the Pokemon was worth the price (and if I wanted it, but that goes without saying.) I would probably buy it.
And considering how I think if Pokemon was real I highly doubt that anyone could make a living on training alone, as I also question whether or not people in the real world would be so willing to pay you for beating the crap out of their Pokemon. As such, people would have to get real jobs, among those being Pokemon sale. So, if it was a trade I was good at (and something I liked doing.), I would probably sell Pokemon for a living.