I don't know why people leave feedback for accounts just because they are bought /sold. It doesn't really accomplish anything in the big picture, and only serves to drive the activity underground, which leads to a lot of people getting screwed over. If people didn't leave negative feedback, people would probably be a lot more open about what accounts are being sold/bought, which would be much more beneficial over the long term than the short term benefits of a few accounts getting bad feedback.
Personal experience. Every single purchased account I've ever encountered was done so for reasons that were not exactly above-board, to say the least. I'm sure there are a few cases where people use them legitimately for sig campaigns, but as a rule of thumb I do not trust purchased accounts.