Your account must have very good reputation so that they want to buy it, and what for after that purchase?
You can easily guess that what they want with your trustworthy username and account, it's mostly for scam.
Thinking about it, morally principle action will result in your refuse to sell that username/ account.
If I sell my account, they will use my name, contact my friends and other members, and ask for money. If I have a good reputation on the forum or any other online platform, they will go there and ask for money or scam people in different ways. But the scammers actually do not want to buy any username. They just created a new telegram account and used a similar username, so people believe they are the original one.
I believe you saw the thread LM created lately. As for OP, it was not an attempt to buy their username, but it was a scam attempt. The scammer would make him pay some fees in TON, convincing him that his username was too valuable and that he would get hundreds of TONs if he sold that.