Ebay's feedback system has become a bad joke I don't even bother with. When I have a problem with a seller I just file a paypal claim and leave it at that.
Remember, just because someone has the $ to file a lawsuit doesn't mean he will win it. "Mere opinion" is not a basis for a defamation or trade libel claim and truth is a defense. The plaintiff still has to prove a case.
What's really going on when a business targets a critic often is a strategic lawsuit against public participation--we call it a SLAPP suit--which is litigation filed to scare people into not expressing views that might damage a commercial interest. The theory is that if you sue a vocal critic and subject him to ruinous attorneys' fees and costs you can scare everyone else into not commenting. A case like that filed in CA is subject to a motion to dismiss on the basis of CA's anti-SLAPP law, which provides for an award of attorneys' fees to the prevailing defendant. I handled a case like that some years ago for a N54 member:
http://articles.nydailynews.com/2005...lifornia-judge