There are two strategies of bidding, depending on how aggressive you want to be. The first is to set your true maximum bid right when you've decided you want something. The other is to hold off on the bidding until the last minute and decide then if you can still get it for a reasonable price. Once you come across a Dutch auction , make sure your read Ebay's information on it because it is different. I'd say that Dutch auctions account for less than 5 percent of all auctions.
All auction sites have rules on how to cancel bids and the strict rules against doing so.
Make sure to read the rules on bidding before you bid.