Five percent of survey respondents said they had bid on or bought counterfeit goods. That would come as no surprise to Tiffany & Co., which has sued 50 sellers and eBay itself, alleging the company was complicit in allowing the sellers to “infest” the site with poor-quality fakes.

But it’s not just pricey goods that are being counterfeited, and the problem isn’t limited to eBay. Lisa Lyttle, complaint supervisor for the Internet Crime Complaint Center, recommends being especially careful in any online auction of repossessed cars, purebred pets, sports memorabilia, tickets for major sporting events, or hot video-game systems during the holidays.

eBay told us it has partnered with law-enforcement agencies and 18,000 trademark and copyright owners (the companies behind the name brands) to rid the site of counterfeits. Recently, it also restricted sellers of luxury goods, requiring verification of bank-account information, limiting the number of items sold (legitimate luxury goods aren’t typically mass-produced), limiting cross-border sales, and eliminating luxury-good auctions of three or fewer days, which could end before authorities notice.

Think twice before bidding on new designer goods. Frederick Felman, chief marketing officer of MarkMonitor, which scours auction sites for counterfeits, points out that top-shelf brands don’t generally sell on auction sites at bargain-basement prices.