It's not "what could a buyer say" that got him banned. It was hitting him with several negative feedbacks in a short period of time from several accounts. This quick hit and run tactic is quite common to knock competitors off ebay. I used to sell a lot on ebay and knew many other sellers. Heard of several people that had this happen to them. IIRC once if your seller rating percentage got to a certain low point you were kicked off.