Something written directly in your avatar, like this user did: https://www.steamtrades.com/user/76561198105197441
By the way, the guy I linked above just posted a warning on his activity feed, to warn others about the same scammer.

You will notice that, as your rep keeps getting higher and higher, you will get more impersonators.
The new setup of Steam Trades should make things more difficult for impersonators, although they usually target the less informed people, so it will probably just make things easier for those who wouldn't fall for them anyway.