Quoted from Damonator:
You are correct - it's when the seller buys back his own pin. Buying it back via bidding allows him to bid a potential buyer up to his reserve...whereas a reserve would simply not be met when the last bidder outbid the other legitimate bidders. Buy backs suck, but so does losing a lot of money as a seller if there is a small turnout for the auction. I can see both sides of it.
I've been on both sides of this equation, albeit many more times as the bidder than the seller. As I have stated many times before I am in favor of the buy back system. They clearly advertise that is what can happen if you listen to them going over the rules at the beginning, or bother readying the thing you are signing to get a bidder number. It's also on the web site. So nothing underhanded going on.
They could do reserve prices or starting bids, but I like to believe that what an owner "needs" could be very fluid depending on how the auction is going. To me it's more sporting to just let them bid on it as well. I have never let one of them trick me into paying a singled dollar more than I wanted for an item. But because it is a sport I can say many of them have re-loaded games they have brought when they were bidding against me and they got too greedy.