The people who appear to be bidding against themselves aren't. Bidder 1 puts in a max bid of 100, but currently the bid is only at 50. Bidder 2 puts in 60 and is outbid. Puts 70 and outbid. 80 outbid. 90 outbid. 100 outbid. 101 is new bid and they are the current high. It looks like they've increased the price on themselves but they haven't. They just bid in increments until they are the high bidder.