And also for sure, there are some people who bought lot of AWS credits or some coupon inside there are some AWS credits.
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.
Purchasing aws credit in order to save money from different services. It's legitimate though, buying that would let you save some pennies.
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.