There's nothing wrong in selling telegram username if it can fetch you some money. The thing is, if you have a unique username and you wish to sell, do not listen to PMs from a random stranger on telegram. Go to fragment and put it up for sale. Anyone that contacts you directly on telegram is a scammer and wants to trick you into clicking or visiting a phishing website where your wallet will be drained.
This is another opinion of it. Merely looking at it, I would have just concluded that selling telegram username is wrong.
What is the possible reason that could make someone sell their Telegram username?
I already know that contacting me about anything will give me the scammer vibes and if there is a link to it that's even worse. I think Telegram scammers are easy to know because the way they operate are all the same.