I dont believe the Karma thing is necessary either.
If anything, a trader rating could be benificial. If you buy something from someone here and they are genuine, then you add to their trader rating. Those with a few hits on their name will 'appear' to be genuine fellows and thus their post will have the respect of their peers. You'll never stop dreamers from signing up and posting here, but its the regulars that act like pitbulls and bite in their responses that lets this forum down. If no one responds to them, they'll get the message and leave.