I'm not sure if you could find a trusted escrow to assists you here since no one can verify an egiftcard if its still unclaimed without claiming it so how could the escrow verify the giftcards? same thing with the activation codes
Well the buyer of the giftcards send the money to the escrow, when the escrow has received the funds, the escrow can give the codes to the buyer. Than the escrows waits for confirmation that the cards work of the buyer and he releases the funds to the seller. It is as easy as that.
What is the buyer says that the code isn't working and has already been redeemed? There would be a dispute then. The escrow would have to trust the buyer then and unless the seller has bought the gift card and has got proof from the website that the card wasn't redeemed before that, the dispute wouldn't be settled for long. Only few websites like Sears allows the escrow to check if the card code is still valid and the value while amazon doesn't have this option.
I have not thought about such a case before, this would mean that the buyer is lying to the escrow and therefor is scamming the seller with the use of an escrow. Probably escrows won't bother to deal with unverifiable giftcards than.
@OP I think you have a bit if a problem than, there is no such trustless trading platform (yet).