Thanks for writing a well-detailed thread. I believe most of the people are seeing this for the first time. I have never seen such a scam attempt, and it never happened to me so far. But who knows? Probably one day, I could get a similar offer in my private message. Probably I would try to figure out what is happening, but since I already know what is going to happen, I will be cautious now.

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.

Check this thread https://bitcointalk.org/index.php?topic=5534693.0