Some people also bought a large amount of that when there are some discounts, so something like they will hold those lot of AWS credits/coupon/codes.
Then they can sell it with someone with the best price, like a win-win situation for those buyers and sellers. That's why you can see some sellers of AWS code are have much better price than buying in someone.
If you are a regular paying customer of a hosting site it will help you a lot on paying the service rent by applying coupons and getting a discount from it.
Buying and selling the codes can be more profitable.