Bargain hunters beware: Your online auction purchase might be stolen goods.

Retailers estimate that one-third of auction and blog sites' listings for "new in box" or "new with tags" items are actually goods that were stolen through organized retail theft or otherwise fraudulently obtained, according to a new National Retail Federation report.

Nearly two-thirds—61.1 percent—of the 76 retail loss prevention executives surveyed reported seeing an increase in such "e-fencing" over the past year.

"Thieves are preying on people looking for the best price," said Rich Mellor, senior advisor for asset protection at the NRF.

What thieves pick comes down to two attributes: "small box, big value," said Mark Turnage, chief executive of OpSec Security, which has monitored online sites for stolen items on behalf of clients. Commonly stolen items include razor blades, makeup, skincare products, baby formula, over-the-counter medications and tooth-whitening strips. Thieves also lift small gadgets such as disposable cellphones, digital cameras and electric shavers.

Gift-card buyers may be an unwitting link in the retail-theft chain, too. About three-quarters of retailers told the NRF they see thieves returning stolen merchandise to get store credit, which the thieves then sell on the secondary market. Shoppers would encounter those as secondhand gift cards, probably loaded with an odd dollar amount.

Even if you're not concerned about the long-term, broader effect of organized retail theft on your wallet—retailers may account for such losses when setting prices and determining discount promotions—there are plenty of reasons for shoppers to worry right now about whether they're buying stolen goods.

Health and safety concerns are paramount for ingested goods—including over-the-counter meds and baby formula. "You have no idea what the storage conditions have been," said Turnage. Products may be expired, or have been stored at temperatures too high or low, making them less effective and unsafe to consume, he said.

There's also a slim chance of legal consequences for possessing stolen property. "Certainly, theoretically, someone who buys stolen property from an online vendor is just as liable as someone who buys it out of the back of a truck," said Stuart P. Green, a law professor at Rutgers University. Depending on how the local or state law is written, not knowing it was stolen isn't always a defense.

That said, the odds of being arrested are low. "As a matter of enforcement, it's bound to be harder to pursue people who are doing that," said Green, author of "Thirteen Ways to Steal a Bicycle: Theft Law in the Information Age." Law enforcement is more likely to go after the thieves themselves, as well as the websites facilitating the transaction.

Spotting a steal

Retailers have been working with law enforcement and various websites to reduce the number of listings involving stolen goods, Mellor said. An eBay spokesman said the company has dedicated teams to mitigate listings of stolen property and other fraud. "We utilize a combination of sophisticated detection tools, enforcement and strong relationships with brand owners, retailers and law enforcement agencies to effectively combat fraudulent activity and present our customers with a safe, trusted shopping experience," he said.

Still, it's worth approaching online purchases from unknown retailers and individuals with caution:

"The very first warning flag that should go off for the consumers is quantity," said Turnage. Someone selling one or two new items might have legit reasons—they were an unwanted (and unreturnable) gift, for example. But thieves tend to list in bulk, something you'd spot looking at that seller's other current and recent listings. "Nobody goes into a store and buys two gross [i.e., two dozen dozen] and says, 'Oh, I bought too much, I'll go home and sell them online,'" he said.

Big discounts of 25 percent off or better can also be a red flag, said Mellor. There shouldn't be the same price cut on a brand new item as for one that's already been opened or gently used.

It can also help to scrutinize packaging, in photos and (if you do choose to buy) upon arrival. Thieves often blur out existing expiration dates or re-sticker them to make the product look current. But this tactic isn't foolproof. "The thieves are very good about changing labels on product," said Mellor.

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