Become an auction stalker and leech off someone else's searching skills.
Often the best deals on eBay are the auctions that most bidders don't find, usually as a result of sellers not knowing what they're selling or not taking the time to promote them properly. The better you become at searching, the more likely you are to find the auctions that are off most bidders' radar. Sometimes, it takes nothing more than dumb luck to stumble upon a great find; occasionally, it helps to rely on other users' dumb luck (and skill) as well.
As much as eBay is a single community of millions of users, it can also sometimes feel like a bunch of micro-communities, each centered around certain genres and auction categories. As you use eBay and become more familiar with the categories in which you're interested, you'll start to recognize individual buyers who, like you, frequently return to eBay in search of more antique pottery, model trains, Ford Model A restoration parts, first-edition Hemingways, or whatever else you might collect.
As soon as someone bids on an item, that bid becomes public record, even though the bid amount is kept private until the auction ends. All you have to do when you see that someone has bid on something in which you're interested is search for other auctions on which he has bid. Not only will you discover auctions for similar items, you'll discover new items that you may not have even known to look for.
 The exceptions are private auctions and auctions held in localizations with strict privacy laws, such as eBay Germany (www.ebay.de).
To do this, just highlight the bidder's user ID, copy it to the clipboard (Ctrl-C), go to Search By Bidder, and paste (Ctrl-V) into the Bidder's User ID field. Select No to exclude completed auctions, and click Search. You'll then be shown a list of all public auctions on which that user has bid, whether or not he is the high bidder.
It's typically in your best interest as a bidder to have as few people as possible see an auction on which you're bidding. Fewer interested bidders means fewer bids, which, in turn, means a lower price and a higher likelihood that you'll win the auction.
Bid shadowing is not common on eBay, but is practiced by some of the more determined users from time to time. The best way to prevent others from shadowing your bids is to bid later in the auction, thereby shrinking the window of time during which other bidders can see where you've bid. See [Hack #21] for a way to take this to the extreme and effectively eliminate shadowing altogether.