There's the other side to that great thing about gift cards coin. We were given a bunch of gift cards about eight months ago and still have not managed to use them yet. A couple of them were sold on eBay, and the others we figured we would be able to use. Unfortunately this has not yet been the case.
First example: $100 gift card to Circuit City. We do electronics, computing, video games, so it seemed plausible that we would be able to find a good use for the card. But we are always looking for a good bargain, we prefer to buy things used, or we will only buy something new if it is offered on sale at a reasonable discount. We rarely have retail store loyalty.
Another example: $20 Barnes and Noble gift card that I've had since Christmas. Definitely an appropriate gift because I regularly purchase books. But I buy them new in stores because used books are just as good. I've been to the bookstore with gift card in hand many times, but have been unable to buy anything at retail price knowing that it's much cheaper (and recycled) online.
Luckily, existence of a market for gift cards and certificates makes it so that if I ultimately decide that I'd rather have slightly less cash, I can part with the gift card online. Similarly, if your niece/nephew is that that into the Radio Shack gift card you got them for Christmas, they can barter for a different sort of gift card or trade it for universally-accepted cash.
Note the following restrictions for selling gift cards on eBay:
* The value of the gift card may not exceed $500.
* You must have the gift card
* You can only list one card per week
Also, you are likely to lose 10% of the card's face value + your eBay listing fee.
Bankrate.com provides more info about the secondary market for gift cards, including various other venues for swapping and sales.