Couple of questions,
If these traders are banned, then surely that solves the problem.
If they are scammers, they won't use the same name or email address, so how could anybody 'avoid' them?
How about doing something to the FS / FT board so that you can't post a new topic without some kind of clearance from a moderator? That way a moderator gets all the relevant info before a poster can post, they could do a very quick check on the info provided by the poster, but it would still be up to the buyer to check the seller out properly.