Mike I think a Sticky thread should be started in that forum. For those who are repeat abusers who have a reputation for being involved in the bad transactions that have been occurring frequently post a link to the Sticky thread and list the thread along with the members username. This sticky should remained closed so that only yourself or the moderator's can post to it, at least this way the information will be easily available for anyone who wants to find it.
The I-Trader has some holes in it, if you give some one a bad rating even if you are right you will also receive the same back, this affects some people decisions because they are new and have no other reputation. Also some members in the past have had friends add to their I-Trader rating just to build it up with no transaction really being made, so this kinda makes I-Trader not as effective as it was intended to be.
Mike, I think the sticky thread would be a good place to start. With only yourself and the Moderator in charge of the forum adding the information I think people would not be able to string so many members along like they have in the past. Also the information in the sticky would only reflect facts that have been substantiated not what some ones friend or enemy can post to either cover their buddies ass or destroy some ones reputation.
In my opinion this would be a simple and a good start. I think it would alert members more accurately than any other method we are currently using. Whether the names / threads stay there permanently is up to you, if were up to me they would never be removed because people do not change these habits easily.
JIMO