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.
A lie is always a lie no matter how sweet sugarcoated it is. A little lie can cause a whole disaster if care is not taken. When it comes to trade, it shows how unstable and unreliable someone who tells lie just to try convince someone to get in a project just for they telling lies to benefit. That is too dirty of an individual. I believe sometimes people tell lie just to cover up something's for their friends or loved ones but no matter how tough the situation could be telling a lie could end up excavating things beyond what one could imagine.
I have never seen anywhere lie after being told does good but rather harm the situation making it escalate.