One important tip for buyers: Stick with legitimate vendors. Many purchasers get their cards from the retailer itself, banks and serve-yourself racks in grocery, office and convenience stores.

But be wary of buying cards at online flea market sites or from suspicious websites, says Dennis Garrett, associate professor of marketing at Marquette University. "It's the Wild West out there," he says.

"You have to do due diligence as far as who you're buying from," Garrett says.

Avoid huge discounts, like a $500 card for just $80, says Daniel Butler, vice president of retail operations for the National Retail Federation.

"Ghost sites," which are "exact copies" of legitimate retail sites, are another concern, Butler says. They're really after your bank and credit card numbers.

If you're buying a card from a rack, make sure the packaging is intact and, if there's a scratch-off personal identification number, ensure it hasn't been disturbed, says Rea.