Snipers essentially convert the ascending-bid proxy auction used in eBay into a Vickrey second-price sealed bid auction, allowing a buyer to not reveal their preferences to other participants. In theory, with rational participants, this shouldn't have any effect on revenue. In practice, buyers do not always understand auction mechanics and delay setting the highest price they're willing to pay until they are outbid. If they're outbid 3 seconds before the deadline, they lost.

If you set the highest price you’re willing to pay it inevitably drives the price higher up then it would otherwise go. There’s something about putting in a bid and immediately seeing that the price has increased that causes the person to bid again.