I often use this site and they give 15%, even though sometimes their service is really slow.
I can confirm this service was legit when i buy amazon gift card a month ago.
I doubt they are legit with that kind of discount, and there's also his negative feedback on Localbitcoins.
https://localbitcoins.com/accounts/profile/Geeney/feedback/negative/My guess is that he is (re)selling gift cards bought using stolen credit cards, selling the same codes multiple times and/or laundering money. Amazon does not seem to withdraw money from your account but they may cancel the GC if you don't redeem it quickly.