This article is from the eBay FAQ, by William B. Shaw toppsoft@yahoo.com with numerous contributions by others.
Essentially, when you bid, you aren't telling eBay how much of a bid to
place. You are actually informing ebay as to the maximum amount you are
willing to spend on this auction. The actual bid placed by eBay's bidding
system is either the minimum amount for the auction (if you are the first
bidder) or one bid increment above the previous high bidder. In the event
someone else bids, eBay will increase your bid up to your maximum to keep
you one bid increment above the other bidders. If another bidder exceeds
your maximum, they become the high bidder at no more than one bid increment
above your maximum bid. EBay gives some very good examples at
http://pages.ebay.com/help/buyerguide/bidding-prxy.html.
 
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