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.