Though the above posters are probably correct, there's another possibility that hasn't been mentioned.
Maybe the seller just doesn't like these particular pins and couldn't sell them at a "regular" price, so they just want to dump them off for cheap.
I'm doing that right now, myself. There was a pin from a mystery box set that I got several duplicates of and they're just taking up space. Admittedly, it's a rather boring pin and no one seems to want it, so I'm selling all of the duplicates way below what most people would just to get rid of them. (I'm not much of a trader anyway and I won't be doing back to Disney until at least 2013).
Just thought I'd mention the possibility.