Proxy Bidding, perhaps this will help you understand.
eBay uses a helpful bidding system (called proxy bidding) to make bidding on auctions more convenient and less time-consuming for buyers.
Here's how it works:
When you place a bid, you enter the maximum amount you'd be willing to pay for the item. Your maximum amount is kept confidential from other bidders and the seller.
The eBay system compares your bid to those of the other bidders.
The system places bids on your behalf, using only as much of your bid as is necessary to maintain your high bid position (or to meet the reserve price). The system will bid up to your maximum amount.
If another bidder has a higher maximum, you'll be outbid. BUT, if no other bidder has a higher maximum, you win the item. And you could pay significantly less than your maximum price!
This means you don't have to keep coming back to re-bid every time another bid is placed.
Note: This bidding system does not apply to Multiple Item Auctions (Dutch Auctions).
Note: In Reserve Price Auctions, if your maximum bid is the first to be greater than the seller's reserve price, the eBay system will automatically jump the price up to meet the reserve, and bidding will continue from there.