Auction Ninja has a feature called a "max bid." It allows users to enter a dollar amount that they are willing to spend on an item. For example, let's say you are bidding on a dining room table, and you're willing to pay up to $100. By inputting $100 as the bid amount, Auction Ninja will bid on your behalf up to that $100. As other people place bids, they will get an outbid notice immediately until someone bids over $100.
It can be a nice set-it-and-forget-it feature, and it can also be pesky if you're the person getting instantly outbid.
One thing to keep in mind: In the event you place a bid that matches the max bid amount placed by another bidder- effectively a tie bid - the person who placed the max bid is considered the current high bidder.