Before you place a bid, always ask yourself: How much do I really know about this item? Always be wary of words such as rare, vintage, valuable, and collector's item in online auction listings. In many cases, the words will be true. But there are several other possibilities, as well. For example:

• An inexperienced auction seller may not know enough about what she is selling. Perhaps a friend or relative has convinced her an item is “rare,” so that is how she has posted it. She has trusted the judgment of someone who is not an expert.

• An experienced auction seller has posted the item knowing that some buyers will bid on almost anything labeled “collector's item.” The item indeed may be collectible right now, but it may go down sharply in value in just a few weeks, not up, as public interest in the item's category fades.

• A new auction seller has purchased some “rare” items from someone who seemed to need money to pay medical bills. Unfortunately, the items were stolen, and the new seller has listed them for auction without knowing this.

Before bidding, be sure you have reviewed an auction site's list of items that must not be sold or are subject to certain sales restrictions. You may want to print the list for each site where you have registered and keep it handy in a reference binder. On Yahoo! Auctions, for example, the prohibited items cover a broad range, from switchblades to fireworks and from counterfeit stamps to stalactites and stalagmites from caves on federal land.

Someone may post a prohibited item for bid and not be discovered by auction site personnel until the item has been sold or someone has complained. Honest buyers are encouraged to help control auction sites by contacting site personnel and reporting any illegal merchandise that they have found up for bid.

Buying Guides

eBay has simplified the auction research process for new buyers, at least in several major product categories. The auction giant has posted a series of buying guides, covering popular topics such as art, digital cameras, jewelry, and sports memorabilia. The buying guides discuss what to look for in particular items, and they provide quick links to the appropriate auction areas. For example, if your interest is radio-controlled model airplanes, and you are seeking a Cox engine, the Radio Control Buying Guide contains a Shop by Brand section, with a link to all current auctions for Cox engines.

Other auction sites have buying guides. For example, the Web site eSmarts.com publishes an “Auctions Buying Guide” that gives basic auction buying tips, links to several Best Auctions sites, and reviews of a few online auction sites. As another example, About.com's Stamps/Coins page presents a collection of online resources for buying stamps and coins at auction sites and has articles that describe how to determine the value of stamps and coins.

Searching Closed Auctions

One of the most valuable tools for buyers on eBay and other auction sites is the ability to search items in closed auction listings. If you are thinking of bidding on a particular item, you can look for similar items auctioned within the recent past and see their selling prices and who posted the winning bids. Knowing the price ranges in completed auctions can help you stick to an upper limit when you enter the bidding for a similar item in a new auction.

You may also be able to read questions posted by potential bidders and the seller's responses to the questions. The information in these may help you learn more about the item you are watching.