This is the problem with selling Gift Cards, especially Amazon gift cards because afaik there is no way of checking the card that whether it is claimed or not other than try to load it up in your account, if it does, its ok, if it isn't then nothing can be done about it. And even if you send it first to escrow, they cannot guarantee that it is claimed or not, because to do that they'd first have to load them up in their account, but they will not be able to issue another gift card from gift card balance.
And this is why it is a risk to both buyer and seller, a buyer can claim the gift card and say that it was empty and the seller can also sell bad gift card and escrow cannot provide cover for this. Which is why I think Bitify (formerly Cryptothrift) has now stopped sale of any gift cards on their website because they cannot resolve if a dispute arises.
One can always get to know when the code was claimed from Amazon or any other website's support department.
If the buyer is the first time holder of the code & not a reseller, using an escrow is not an issue as escrows here are honest and they wouldn't use the code. Escrows who cannot verify the codes only take charge of sending the code to the seller and the seller then needs to verify the code by adding it to his/her account instantly. Note here that the seller shouldn't be a one who resells the code. Problem does arise when more than 1 buyer or seller is involved but the escrow isn't an issue.