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.
It might be wrong to you, and might be okay for some people, so situations like this are a personal decision, and anybody who feels like selling has a reason for doing so. The only thing to be worried about is that the buyer should not use it for illegal activities that might likely implicate you, the previous owner, because if it happens that you are already known as the owner of such name or account and you have been very much active and you decide to put it up for sale and and another person purchased it with ill intentions, it is likely that you might be the one to suffer the brunt because you are the initial owner but I do not know if telegram do send a prompt of change of ownership to contacts already associated with that name or account for their awareness so that in the future if anything happens, nobody would bother the previous owner of the account or username.