It is very simple to realize that it is scam.
From telegram to Fragment websites, if people are simply careful, they won't be scammed. Being directed from Telegram to Fragment that I don't know what it is, people who were victims of this scam, are very careless practically.
For those who have never traded their usernames on fragment it is not as easy as you say. You may have the idea and knowledge that username are getting tarded on fragment so it's the valid platform. But you may never know that the person is scammer.
And it's not un usual that your getting offer for usernames. A lot of people do buy username for their business/personal liking . If your username not listed on fragment they won't be able to bid it. So that particular individual have to ask you to list it on fragment so you both can tarde.
This is my first time hearing that someone can trade their telegram username on fragment. It sounds surprising to me that someone could offer such.
If I ever receive any pm about that, whether the person is real or a scammer, I will quietly block the person asap because I can imagine selling my telegram username for pay when I know what my username will be used for to scam people. Someone can't offer to pay for a username without having a bad intention of getting more than the money they used to buy the telegram username.