Oh, I see. My take on this is that whether the offer from the scammer is genuine or not, people should generally desist from selling their usernames on Telegram. It might actually be a smooth transaction between the buyer and the seller, but others who know nothing about this exchange might become victims by trusting a total stranger, thinking it's someone they used to know. When these strangers come to our DMs with this kind of offer, the response should be "no, I don't want to sell my identity" and not just "oh! This dude wants to scam me." Even if the offer is genuine, don't sell. This way, we would save lots of people from becoming victims. Unfortunately, some persons would be too greedy to even think about the negative consequences of such transactions.
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.
Or maybe this offer will lead you into a scam by sending the scammer a phishing link.
We should must be careful dealing with people in Telegram, it's a house of scammers that should be avoided.