Yep, I tried searching them up as well and evidently none of them are actual members. I suspect he will now go with the excuse that these customers were from another forum completely, however.
It's obvious that he's trying to scam here. He's trying to get you to use autobuy, he's trying to get you to communicate with him offsite so he can prey on unregistered users not being able to view his tainted trust score, and he's selling discounted gift cards. All warning signs already. All he's missing is a self moderated + locked thread.
On his thread he actually claims that he accepts escrow, though I highly doubt that is the truth (selly.gg or any other autobuy site isn't escrow). Even if he does, it doesn't mean that he's safe to deal with - the cards that he sells could easily be carded/fraudulently obtained, and as a result, if you are the buyer, you will not be protected even if you do receive the cards through escrow.