If false negative reports were handled on equal grounds as false positive reports, or fake vouches, I think this would be excellent. Ie. If the seller can prove the person is slandering them then a similar penalty would be raised. If not possible to prove, the unverified negative feedback stands ground with the unverified positive feedback and is rated solely on the credibility of the member giving it, just like any vouches for transactions a user has not personally witnessed take place or participated in.
Positive feedback is taken solely on a members word anyways if no evidence is left, why not allow the same for negative feedback as long as the penalty for falsifying is the same.