There are exchange (or currency exchange) websites that you may be able to find them on, just be careful to:
1) Not be scammed, if you can try and use a middleman or escrow of some sort
2) Make sure the giftcard was not paid using stolen money, for moral reasons and you don't want to be banned by Amazon because of chargebacks. If you can, try and make sure you can see a receipt where they paid in cash and the times match.