I have seen this idea on another site similar to CK and I thought it may be worth discussing, seeing as I have had an experience where it would have been useful.
When there is a dispute being sorted out, it might be of use for both parties involved to have a temporary userbar. One that reads something similar to "Caution Trading: Under Investigation"
Staff would have the decision on whether this was necessary on case by case basis.
The warning would hopefully stop any further people getting involved in the event that said people are scammers and could be removed as soon as the dispute was closed.
Thoughts?