1. Luxury Goods

Sure, you can find good deals online, but how about a Rolex for $50? A Prada handbag for $150? If you believe you're getting the real deal when you spot such steals, there's also a bridge for sale in Brooklyn that you might be interested in.

Counterfeit goods account for about 13% of Internet purchases, says Stephen Polinsky, vice president of sales for GenuOne, a security technology company specializing in brand protection. "There's a real chance for an item to be misrepresented," he says. For example, you'll spot plenty of trendy Louis Vuitton Globe Shopper handbags on eBay. The photos, pulled from the brand Web sites, are of the real thing. But the real bag retails for $1,280, so what you see isn't going to be what you get when the sale price is $100.

Many luxury brands don't allow online transactions, period. Head to the Web sites of two high-end watch manufacturers, Breitling and Corum, and the first thing you'll see is a warning -- genuine products are not sold on the Internet. What you're getting from sites like Overstock.com and SmartBargains are grey-market products that are missing key components like a serial number or the manufacturer's warranty. Without those, the brand will refuse to repair or service your purchase.

Getting around it: Put simply, know the item and the retailer. Getting familiar with the little details is the easiest way to separate the knockoffs from the real thing. To find authorized retailers, visit the brand's Web site and check for store locations.