Can we assume that on another id a buyer has had the temerity to leave you a neg? I am all for contacting sellers if there is a problem but not if they have silly, unenforceable terms in their listings such as: All sales are final, Negs will be left for non payers, Payment must be received by yesterday, Not responsible for items once posted etc etc etc.

What would be the point of contacting them as they are obviously clueless. If I have to open a Paypal dispute to get a refund or I have to pay return postage for a not as described item then of course I am going to leave a neg to warn future buyers, remember the truth is an absolute defense to slander or defamation.