Well I finally got a response after I left him feedback haha:
It seems that you have found his weak point, because he obviously knows how the trust system works considering how he reacted. However, there is one positive thing in everything - now he knows that the forum has its own detectives who can very easily find out who is sending merits to whom, and also connect it all to possible alt accounts.
I figured he might at least own up to selling merit rather than hacking accounts. I wonder if his account is actually hacked? It's an account from early 2018 but has long periods of inactivity. He mentions he's based in the UK in an earlier post but the messages he's sent me are in broken English, or at least it seems English isn't his first language.
I'm not an expert in such matters, so I'll leave it to someone else - although it is possible that someone really lives in the UK, but that he is an immigrant whose English is not his native language, and this can especially be seen in written expression. Perhaps it would be easiest to detect whether the owner has changed through IP logs, but if I'm not mistaken, only admins have access to this data?