Are the giftcards outside of gtm? If so, I don't think this deal is allowed. The only type of donor deal you can do on GTM is in-game items/money for items on the GTM store.
Extra money is not left behind on giftcards. Don't lie, I repeat, using giftcards outside of gtm is not allowed.
Is it that hard for you to understand? He bought something with his 2 GTM giftcards and had 1$ leftover on both the giftcards. He then sold those 2$ to me. I can still redeem those 2$ on the GTM store using the codes he sent to me.