If you think logically, why would someone send an offer to you to buy your username? They could create another account and set a similar username or a unique username. Even if you successfully sale it, you should know that they will start using it to reach your friends and scam them.
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.
Otherwise, they try to scam you especially if your account is not trustworthy and useless for using to scam others. They aim at you to scam and get money from you, while they know that your account is useless.