I've only heard of one incident that was even questionable, and that was born of differing expectations. A friend sold a lot of his old Dragon Magazines a few years back to Noble Knight Games for pennies on the dollar, essentially. NKG then turned around and offered them for sale at over seven times the value my friend received for them. When my friend saw that, he felt cheated.
Ultimately, the problem was that my friend didn't realize the value of what he had to offer NKG, and was surprised at the mark-up. Like any business, NKG is out to make money, of course. Just be aware of the value of your goods before you enter into negotiations.
Hope That Helps,
Flynn