In my selling endevours on ebay, I've come across all sorts of people. Most honest good and greatful.
With the others, I get them out of my life and thoughts as soon as possible. I deal with their issues, ban them from buying from me again and move on.
Dwelling on others decietful and dishonest practices to the point of looking for revenge in not healthy.
If they are getting you down and losing sleep, they are winning.
You don't want to back down due to financial loss or the principal but sometimes you need to step back and take a long look at the situation and how it is affecting you.