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.