Gift card sites such as PlasticJungle.com and Swapagift.com let consumers buy other people's unused gift cards. With Plastic Jungle, customers can search for cards by store brand, amount or discount percentage desired, says Kristin Donelson, vice president of marketing for Plastic Jungle. Typical discounts are in the 10 percent to 15 percent range.

Card-swapping websites are likely to offer a guarantee, so buyers will know the cards are legitimate, says Donelson. With Plastic Jungle, if a buyer purchases a card and for some reason it doesn't work, he may be able to replace it with one that does, or his account could be credited for the amount paid.

The balances of gift cards on Plastic Jungle are sometimes in odd amounts. For example, the value of one card recently offered was $29.16, instead of a flat $30.

This could be awkward if the card is meant for a gift. But Donelson says an uneven amount is usually not a problem, since many shoppers buy discounted cards and then use them to purchase actual gifts. "They use the cards to stretch their holiday budgets a little bit further," she says.