I have tried to do this legitimately for unwanted gifts and its a lot of work. Posting a gift card on the normal buy and sell sites attracts every scammer in the country who all pretend to have sent payments or send fake paypal emails to you and get angry when you don't hand over the code.

And then even when they do pay you they could always tell paypal that the code didn't work.

There's sites you can sell them to, rather than individuals. I've sold lots to a few different sites and I've never had any issues.