I don't think people should be allowed to leave negative trust ratings unless they can prove that the person is a scammer, or they can prove the person is leaving false trust ratings.
This would not be a good idea. It would make it more difficult for the community to stop scammers from scamming in the first place and would make the entire marketplace more vulnerable to scams. If the marketplace is vulnerable to scams then people are not going to want to trade in the first place, which means that overall bitcoin commerce will decline