As an addendum to this, if you are buying a gift card at less than cost price, then you are almost certainly buying a gift card which has been stolen, or bought with stolen ("carded") credit card details. If this gift card is reported as being stolen or fraudulent obtained, and you use it, you will obviously lose the value as the card, and also run the risk of losing your account. With Amazon this can also mean a permanent blacklist of your address and other accounts details (email, credit card number, etc.)

If you are going to trade bitcoins for gift cards, best to stick to a big retailer such as Bitrefill.