Hey, not sure if it's too much, or going overboard, but instead of having just one forum 'Completed Transactions' Maybe make three new ones? Let me explain.
Rename Completed Transactions to Transactions.
Make two new sub-categories in the Transactions forum.
One called Completed/Successful Transactions.
Another called Failed/Scam Transactions.
That way, it divides the 'trustworthy' sellers from scammers.
I wouldn't consider deleting/pruning the banned members threads or trashing them, as other people can see them and known not to do business with them in other forums, etc.