Auction sniping is when someone waits until the last possible moment before an auction ends to enter a bid, with the hope of outbidding other bidders. Sniping does not currently violate any Listia rule, although some users may frown upon it.
Because of how Listia's proxy bidding system works, the best protection against snipers is to bid with the highest amount you're willing to pay for the item. Such a bid will not necessarily cause you to pay more when the auction ends. More details about this can be found here.