Maybe you’re planning a large purchase or a big shopping trip with a certain retailer, and you seek out gift cards to that store from online gift card marketplaces. Swapping gift cards online is a unique opportunity for savings, but also provides a unique opportunity for fraudsters.

How’s that? The sellers aren’t misrepresenting the amounts on their cards or selling counterfeits. According to a report on Krebs on Security, there’s a problem unique to national retail chains: shoplifters turn ill-gotten merchandise into cash by using gift card exchange sites.

The sites aren’t parties to this scheme, but the existence of sites where people can sell gift cards for prices under retail provides an opportunity for them to convert cards into cash easily before card buyers and marketplaces catch them.

One flag to look out for: the reader who initially submitted this story to Krebs purchased Petco gift cards that turned out to be merchandise return cards, not gift cards as they were advertised. If this happens to you, don’t just shrug and go shopping anyway: notify the gift card marketplace.

The Role of Phony Returns in Gift Card Fraud [Krebs on Security]

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