Auction trick

I heard of this one from a talk by a proud startup owner, but I'm sure it's one of the oldest tricks in the book.

In an auction of non-unique items, keep track of the losing bidders, and offer to them, in private, lower price for the same good they bid for, at the price they offered to buy at during the auction. This increases the volume of sales but optimizes the price.

Without knowledge of it, all participants in the auctions are at loss.
Ohh... BTW, the startup, making millions, actually does this automatically while the owner sits there and watch his eBay store earn money.