My negative trust for a member, who just posted a quote with no further comment, was based on the fact that he was attempting to cheat the activity and merit systems in the forum, and this indicated a person of low moral fibre in my opinion. This could be an indication of willingness to cheat bounty managers and other members in the future.
You may be right about this but still, the fact that the trust system is MUST only be used for transactions we can't further do this things anymore. I'm also against that, plagiarizing has the permanent ban, so we should stick also with tagging these humans that lacks intellectual respect for the forum. Giving such thing on their accounts will give the Reputation Section a lot more use, for they can appeal for their Red Trust just like the other people whining being tagged.
And for the case of giving positive trust, I think if you have done a legit transaction you may give that user only ONE positive trust then after that if you will have another successful transaction you may just give him a neutral trust instead of giving another positive trust.