---xx---
A buyer can simply tell the escrow service for example The gift card is not working and has no balance after using it and since the escrow guy have no idea if the card was valid or not he will just refund money back trust me.
---xx---
Seller will send code first to the escrow = he will check the code then escrow re-send the code to buyer.
/cards have no online verification option and yh escrow must verify all deals witch they dont and they cant for some items.