With 100 million items for sale--everything from a 99-cent fruitcake recipe to a $235,000 Ferrari--eBay is the king of online auctions. But almost half of eBay buyers surveyed by the Consumer Reports National Research Center said they’d encountered deceptions. Some sellers took their money and ran, failed to disclose key details about merchandise, or overstated its condition.

Our findings echo those of the Federal Internet Crime Complaint Center, which last year logged roughly 39,000 consumer complaints about Internet auction sites that were serious enough to merit criminal investigation--more than any other type of online fraud.

Deceptions aren’t the only pitfalls. Although eBay prohibits trade in illegal goods, buyers can end up, knowingly or by chance, with unsafe products. We had no problem buying lawn darts, a game banned in the U.S., and we tracked down car seats, strollers, and other kids’ products recalled for possible safety defects. Buying used children’s products might be a poor idea for other reasons. One in five survey respondents who did so reported problems, notably excessive wear.

eBay has 2,000 staff members policing its site around the clock, but with 6.4 million new listings per day, employees can’t find all iffy auctions and shut them down instantly. To help you play safe and win, we recently asked more than 2,500 subscribers to ConsumerReports.org about eBay purchases in the past year. Among our findings:

You can’t eliminate risk when dealing with strangers, but you can lessen it.

Check feedback. The feedback forum, accessible from an eBay member’s listing and profile, reflects his or her trading history, and it’s the only way to know whether the individuals you’re dealing with have been trustworthy. Buyers rate sellers (and vice versa) with a positive, negative, or neutral grade, plus a brief summary of the transaction. Matt Halprin, eBay’s vice president of global trust and safety policy, recommends trading with members who have a high positive feedback score. Look for at least 99 percent.

As of May, eBay began letting buyers rate sellers specifically on the accuracy of the product description, their responsiveness (22 percent of those surveyed said that sellers ignored their e-mail messages), shipping and handling fees, and timeliness of delivery. And they can do so anonymously. That should help foil “feedback blackmail,” whereby one party threatens another with unjustified criticism. The new system might have helped schoolteacher Jordan Simmons of Springfield, Va., a survey respondent who bought a camcorder to record the birth of his daughter. When the camcorder was delivered--five weeks after the transaction and too late for the birth--it lacked the promised lens and cleaning kit. When Simmons left negative feedback, he said that the seller branded him as unfair. Simmons got the bad rap removed only after he agreed to buy an extended warranty from the seller.

Avoid shady sellers. Before bidding, determine shipping cost, understand auction terms and return policy, and check whether the seller has changed identities (only 13 percent of survey respondents said they always take that precaution). If the user’s ID has changed within the past 30 days, an icon will appear on the listing page. To check for an earlier switch, go to the right side of the listing page, to seller info, click on the feedback score, then on More Options and View ID History. Any past names used on eBay will appear.

Go beyond the screen name to verify the seller’s identity by e-mail and confirm a physical address or phone number. Don’t be afraid to ask questions or request additional photos of products. If the person doesn’t respond or is vague, stay away.

Review the types of products the seller has listed in the past. If someone previously specializing in toy cars is suddenly selling glitzy watches, ask why.

Comparison shop. To determine whether a new item is a bargain, consult Web sites such as Shopzilla and Yahoo Shopping. Older collectibles generally have at least one reputable source of information (Beckett price guides for sports cards, for instance). Or you can consult an appraiser. Insist that the seller provide proof of authenticity and condition.

Read between the lines. Sellers might dangle words such as “genuine,” but look for terms such as “inspired by,” indicating that the product is a knockoff.

Bid smart. It’s best to establish a top price and stick to it. Survey respondents cautioned about overbidding, reasoning that the object of their affection would probably turn up again. The auction format might work best for items with an uncertain price: collectibles and used designer goods, for starters.

If you choose to bid by proxy, you needn’t track the auction action, and it’s easier to stick to your budget. You place an initial bid, then enter your maximum, confidentially, in the proxy box. When a competitor posts a higher bid, yours is automatically bumped up just enough to put you in the lead. The drill is repeated until you’ve reached your ceiling or won. Proxy bidding might have helped the 40 percent of those we surveyed who lost track of an auction deadline during the past year and forgot to check on their bid.

Be alert for oddball auctions. When Britney Spears shaved her head in a California salon last February, sellers from around the world popped up overnight, claiming to have strands of her hair, obtained through a friend or relative who was there.

Watch out for “snipers.” If your initial offer is in the lead even after several days, don’t be stunned when someone swoops in at the last moment with a slightly higher bid and steals the prize. The tactic, known as “sniping,” is legitimate. There’s often little bidding until the final few hours of an auction.

Keep your reputation. A bid is binding. Retractions by buyer or seller are allowed only under special circumstances. If you back out of a deal, your account could be suspended and your feedback reputation damaged.

Don’t bite on phishing schemes. E-mail messages that seem to be from eBay and ask for sensitive information are designed to hack into your account. Don’t click on any link or respond. eBay’s Matt Halprin says that eBay never makes such e-mail requests. Halprin advises members to download the site’s Toolbar with Account Guard to avoid getting stung. That way, if you mistakenly click on an unofficial link, the eBay toolbar turns red.

Once you’ve won, eBay prompts you through the payment process and sends e-mail messages to both parties. Payment options, specified by the seller, might include PayPal, credit card, money order, or personal check.

PayPal, owned by eBay, is the site’s preferred method and was used for 89 percent of purchases in our survey. Using it is fast, simple, and safe. Both the buyer and seller must have an account (sign-up is free at www.paypal.com) that is linked to a checking account, debit card, or credit card. Linking to a credit card makes it easiest to get the charges reversed if there’s trouble. When you buy, PayPal promptly pays the seller, who never sees your credit-card number. Most PayPal transactions are automatically protected for $2,000 if you buy from a seller with a positive feedback rating of 98 percent or higher; otherwise, you’re protected for up to $200. PayPal offers dispute resolution for deals gone sour.

With credit-card payments, your liability is limited, but not all sellers accept them, and you might not want to share your number with strangers. Some card issuers will issue a random number for limited use. Check out, for example, Virtual Account Numbers for Citi cards or Secure Online Numbers for Discover.

Avoid checks, money orders, and bank-to-bank wire transfers, which are difficult to recover in cases of fraud. Unless you know the seller, don’t wire cash via a money transmitter such as Western Union or MoneyGram. eBay has banned such transfers, but that hasn’t stopped opportunists from trying. Typically, the defrauder poses as an eBay member and offers losing bidders a second chance to win a similar item by wiring funds through a service.

Escrow services such as Escrow.com, approved by eBay, protect the interests of both sides and are worth using for big-ticket goods. The buyer pays the service by credit card, check, or money order; the seller then ships the goods, and the money is released only after the buyer receives and approves the merchandise. Escrow.com charges a minimum of $25.

eBay recommends that unhappy buyers and sellers first try to work out their differences, but that usually failed for survey respondents who tried it. More effective was the online Dispute Console, in which eBay attempts to facilitate a settlement. Complaining directly to eBay authorities satisfied 60 percent of survey respondents who took that action. But the most effective way to deal with dishonesty was among the least-used: filing a formal complaint with PayPal. Although only 23 percent of unhappy respondents took that step, 66 percent of those who did said it worked.

eBay also offers Square Trade, which provides free negotiation or, for $15, the assistance of a professional mediator who looks for a nonbinding solution that’s fair to everyone.

Finally, vigilant buyers and sellers can help keep the site safe. If you encounter unethical practices, notify the Federal Trade Commission (www.ftc.gov) or Internet Crime Complaint Center (www.ic3.gov). Report suspicious items via a link at the bottom of every eBay auction.