Well, that's up to you. We all lie at some time in our lives. The problem is lying too much or some fat lies that cannot be forgiven.
If you believe that a certain person is lying and this makes you distrust him/her for a trade, leave him/her negative feedback. Others will exclude you from their trust list if they don't agree, but this is relative because if they agree with the rest of the feedbacks and not with one in particular they will not exclude you.
If deceit results in material loss to one of the parties, then this will certainly be reflected in the trust. This is unavoidable and will allow other forum members to remain wary of a lying user should they wish to do any trades with him. Although I would not do this even with someone who has been caught in such a deception at least once, but even the absence of such a recall in the trust does not mean that you can't be deceived. So, at least leave a negative trust that you don’t leave, this is generally only a subjective opinion (if there are no mass complaints and a public fact of lies), which is not direct evidence of the user’s lies. Look, some users have a negative trust, the content of the comment is not always adequate.