This only applies to sellers who are also buyers.  Unlike apparently many sellers I do my buying on eBay under the same user name.  This seems, to me, only fair and above board.  However, I have found to my regret that such is not the case with other sellers.  Twice this year I have had to leave negative feedback for other sellers.  In neither case did I do so lightly and without communicating with these sellers.  What did come as a surprise to me, is that both sellers, using other user names, then bought items (cheapies) and then left me negative feedback.  In the last case, they bought a vintage photograph with my usual shipping cost being only the actual postage, shipped the same day as paid.  Yet this person had no problem leaving me negative feedback, and even warned that I was a dangerous "buyer and seller", clearly indicating his revenge for the negative I had fairly left him.  This seemed to me to violate eBay policy and so I contacted an eBay representative and was told that this was not a violation of their rules and that they could not do anything about.  the fact that the feedback, even in the buyer's own words, was not just for that transaction, violation one.  The buyer had not contacted me (for obvious reasons), violation two.  the buyer had not attempted to work with the seller (why should he since there was no problem with that transaction), violation three.  But even three strikes means nothing to eBay.  This clearly shows a basic weakness of the eBay system:  They are but robots and without thought which means for all us that eBay is not a human endeavor, but a machine that meshes all of us in its machinery.  For me, at least, what eBay is about and the only reason I continue selling is the fine buyers out there, many of whom have become friends.  EBay, I am sorry to say, can never be a friend.