Sup. They get the credits from the GitHub Student Developer Pack. You have to verify your information, often done with an .edu email account or from another educational institution. These sellers have access to an unlimited supply of .edu mails or a domain that is luckily accepted by AWS and Github. Maybe you can also buy a few .edu accounts, but they must be accepted by AWS and Github, which is rare. You only get one credit code. Of course what they are doing is fraud, though. Your account may get banned after a while.