Oh, I've seen it rampant on eBay, in certain categories. I had the misfortune of being a seller for a while in a hobby that "took off" when collectibles were the big thing.
I wasn't in to that, had other uses for the "products" - but it was sickening to watch. I didn't pursue it after a few weeks, although I had been counting on the income from selling off what I had collected. Still have all of it, many years later and no way to enjoy it as I once did.
I probably just don't buy in the categories where you have seen this. It is unethical, and that is what I said to this old boy who was bragging about it. I don't think he understood the word.
I usually buy from folks that have a "buy it now" price. If you really want to sell your collections, maybe there is a reputable auction house locally?