I purchased an Amazon gift card from him before he locked his post. I got a code but it had already been used, he said he would give me another one but many days later nothing. I posted a reply on his post but he just deleted it and locked his post
The reason why scammers lock their thread and make them self moderated is demonstrated by your case perfectly - they are able to censor out any sort of negative comments towards their scam, even though it's clear to anyone that no way is selling gift cards at such steep discounts a legitimate or sustainable business model. Essentially, scamming would be the only way that he will be able to
Any time you see the combination of a) Autobuy links b) Self moderated threads and c) Locked threads, please move away. A legitimate business would not try to stop people from asking them questions in public.
Most of these people are already tagged by DT anyways, so check their trust. I can guarantee you that if you see discounted gift cards being sold on autobuy, 9.9 out of 10 times, they've already have a negative trust on their reputation. Even if they don't, don't deal with them. If it's too good to be true, it probably is.