So what is the ultimate goal of the scam? You use a fake website, he pretends to pay, and the website then asks for some deposit to "release" your funds or what?
Unless it's something like that, i don't see any other risk associated with this
Scammers when they first meet a client, don't give the impression of scamming, they keep the chat more formal to make it look a lot legit. Asking him to sell his username is the first part of the attack, it isn't a big deal and poses no threat yet, what they need is for the person to accept the offer, then they go ahead with the next format until they get closer to where they will demand a deposit.
One way to know scam attacks is they make the offer enticing so client should be interested, avoiding unknown user PM or setting restrictions on your telegram is best preventive measure to use.