Why should you sell gift cards? It doesn't make much sense unless you have bought it by mistake
What are you on about?
Amazon gift card code are given as gifts, as payment for work, bonuses etc.
There's a massive online market for this and thousands of these are traded every day on Local Bitcoins alone.
People even buy gift cards to trade as its their only route/option to get Bitcoin.
Trades usually mean the buyer of the gift card obtain the cards value at a discount, that''s why they don't just "buy it themselves on Gyft" if they come across a code being sold for 80% of it's value.
Your post baffles me that you can't understand why people would trade these.
There's your answer.