OP, you already know the answer, we here have three Types of feedback.
feedback
Type:
O: Positive - You think that this person is unlikely to scam anyone.
O: Neutral - Other comments.
O: Negative - You think that trading with this person is high-risk. You might also be able to add a flag.
Besides, you already quoted what was hinted at by: @theymos, isn't that clear enough, what should be given to a liar.
To me, if you make a trade, then the user you made the trade deal with is lying and you have valid proof and you are really at a loss in that case, Negative clearly as an option, it doesn't only happen here, outside this forum, also taking such actions.