I think that we have all felt a need for "revenge" at some time, I myself feel agrieved at the way my local MOT/Garage have conned me into spending more than I needed to to get my car through it's MOT, but I have to agree that it would be foolish (and expensive) to name that Garage.
Sometimes, you have to bit the bullet and accept that on occasion, you will have an incident which upsets you. I look back at the service which I recieved in the past from this garage, and on the whole, I have been quite satisfied. My experiance recently was bad, and apart from voicing my dissatisfaction with the Manager/Owner, I have the option to take my business elsewhere.
As far as having a bad experience with Ebay, that is what their Feedback Rating System is for. Check the feedback befor you buy...If you are deeling with someone with bad feedback, then that is a case of buy at your own risk.