The ebay system doesn't allow you to bid against yourself. If your $10 bid is winning an auction but you're worried someone else will steal it from you, go ahead and put in another bid of $100. If no one else bids, you'll still win it at $10. It's just another way of raising your MAXIMUM bid. It can be useful to get your foot in the door at a low price (so the auction can't be closed) and then, at the last moment, put in a much higher bid.
David Bean
Arlington, MA