What stops a scammer from writing down the relevant card numbers, selling the card to CardPool (or any competitor), and then still using the card after receiving payment?
Second, to protect ourselves, we've developed several methods of detecting high-risk transactions and ways to mitigate them (such as working with merchants to reissue new gift cards). I'd go into more detail, but we don't want the wrong people to get ahold of this information ;).
In the end though, we'll be fine. The fact that we mail the check to a physical address, make out the check to the seller's name, and the seller has to cash the check at a real bank, is already a big deterrent to bad behavior. These bad people will have an easier time trading their fake gift cards to people on sites like craigslist.