I privatized mine after a seller for the 3rd time held my feedback hostage and then left dishonest negative feedback in retaliation of my truthful feedback.
It's a very sad and common practice I'm afraid and Ebay won't do anything about it. Sellers don't seem to realize they need to leave feedback for a buyer's payment only, and immediately after being paid. That's a buyer's only obligation. A confident seller with a decent honestly advertised product wouldn't worry about "having the upper hand". I always left feedback immediately after a buyer paid, when I was selling stuff. Not only is it the right thing to do, but one could also argue that it makes the buyer happy and more likely to want to do the same in return.
So anyway, after I got burned for the third time, I decided to make the lies private..