While I agree with several of your conclusions here, and you bring up very good points about there being no limiting factor on frivolous negative ratings, I think your solution is lacking and is only adding more complication to an already over complicated system. The system needs to be simplified, not added to. My suggestion is to have a standard of evidence of theft, violation of contractual agreement, or violation of applicable laws before leaving a negative rating. This solves the primary issue of there being no limiting factor to false or frivolous negative ratings with minimal effort, creates a simpler system, and less counter productive side effects.
standard of evidence of theft, violation of contractual agreement, or violation of applicable laws just means that people need to be smart enough to not leave certain bits of evidence behind while doing what they are doing. Not being able to leave someone feedback on a technicality doesn't seem fantastic either. In addition, untrustworthy behavior is incredibly subjective, so who will decide what warrants negative feedback?
If someone leaves a frivolous negative because they believe you are untrustworthy on the grounds that you bought your account for example. A fair number of people might find that significant and would rather it not be covered up because it isn't breaking any laws, agreements, and no theft was involved.
What is your point even? If they are smart enough to erase their trail completely then you aren't catching them anyway. Again, what evidence is available would be submitted for public review. Neutral ratings could serve as warnings with the thread referenced. I agree "untrustworthy behavior is incredibly subjective", which is why I am arguing for an OBJECTIVE standard of evidence before negative rating. Account sellers could again be neutral rated with a reference thread. This is not covering it up. This is defining a very objective line so that the community can focus on fraud, not absolutely everything being perceived as subjectively "untrustworthy". The negative trust factory farmers would just have to find a way to get their dopamine hits elsewhere rather than from dropping negative ratings on people assembly line style and fellating themselves over the moderate amount of authority it gives them.