Exactly how is the site going to compensate you? Is the method convenient?

Some sites may offer to make a deposit to your PayPal account. However, if you're not already a user and have to set up the account just for one transaction, it could feel like a hassle.

If you want to trade or sell gift cards, consider whether the site has cards you want. Different sites may specialize in different types of gift cards or merchants, says Mendelsohn.

"Some sites will give you credit to take whatever card you want," says Mendelsohn. "Some of the sites send you a check. Some of the sites will give you credit to shop only with the store they work with. Be careful. Look at the policies for the site before you send off your gift card."

You want a site that makes getting cash or much-wanted cards easy for you, and the site should safeguard your cards and data in the process.

It also pays to be a smart consumer, says Mendelsohn. When you send gift cards, use a mailing service that includes tracking. Save all your e-correspondence and receipts "until you're finished with your transaction and are satisfied with the goods."