I doubt that it's his real name and picture. These scammers are hiding in every detail that they can. Well, if he's going to buy my TG username, I'll sell him that for 8k USDT. 
Afaik, they would trick you into transferring ownership of your username and later use it to scam others connected to you, since no monetary transaction is involved here.
But if they have bought and paid you, that won't be a scam anymore and you could careless what they'll do next. However, the problem starts if they've used your name already so, that's one thing to worry about. They have studied this technique and probably took them a few weeks to strategize. Be careful everyone and thanks OP!