Be very careful. Incredible amounts of gift card fraud on these sites. Seller sells gift card to site, and then once paid, they use the gift card or sell it again to another site. If you're going to buy it, you need to use the card immediately like within 72 hours. If you're buying it for a rainy day, I'd run.
And if you think you'll get your money back from these sites after reporting fraud, think again. You may get it back eventually, but it will be a long fight.