There are a couple of ways in which you can outbid yourself so beware.
From the help part of ebay.co.uk
http://pages.ebay.co.uk/help/basics/e_bid_conf2.htmlIn most cases bidders cannot outbid themselves. There are two exceptions, however.
Let's say you are tied with another bidder, but you are the high bidder because you placed the high bid first. If you place another bid to raise your maximum, you will lose your favoured "early bird" status. As a result f placing another later bid, eBay will increase your bid to one bid increment more just so that you will remain the high bidder.
If your current high bid is less than a full bid increment above the next highest bid, and you place another bid, your bid will increase to a full bid increment above the next highest bid. The proxy bidding system will try to make sure there is a full bid increment between the current high bid and the next highest-bid whenever that is possible. For instance, if you have a situation where your maximum bid is £15.56, but the next highest bid is actually less than a full bid increment lower than that, say £15.25, then if you increase your maximum bid the current price will become £15.75, a full bid increment above the next-highest bid.