There are members with a ruined reputation here but have a really good reputation on p2p sites, what do you think will happen if they're able to carry over those good scores to the forum ? Wouldn't it make the whole feedback system useless?
It does not affect the trust system in anyway, if you make a reputation thread or something similar to that about your perfect reputation elsewhere, but you have a tainted reputation in the forum, anybody who wants to deal with you can see the both and decide if they want to go ahead or not.
That being said, a scammer elsewhere, would most likely be a scammer here too.