This is a situation you cannot escape no matter how good of a seller you are.
I myself have a negative on one of my accounts. I was working my real job one day. A 10 hour shift. Came home to a negative feedback. Apparently the buyer sent me a message at 1PM I got off at 6PM But At 5PM they left me a negative stating I never returned thier message. LOL
About the dumbest reason to get a negative. Apparently Im supposed to be sitting at my pc 24/7 watching my inbox?
Well anyways you cannot escape with a 100% rating. Just move on leave them a negative in return. It will not kill your buisness to have negative feedbacks. unless you have too many. Just do your best to have successfull transactions. Offer refunds, Offer replacement guarantees and always respond to your buyers in a professional manner no matter how they message you.
I have so far done 4,000 Plus transactions and have only recieved 1 negative. I also have it stated clearly in my ME profile that I will respond with a NEG for any NEG left for me. So it sometimes will deter most from dropping a NEG on your account.
Dont start new accounts. You work hard for your feedbacks. New accounts will just slow down your feedback growth.
And as gulfy stated. Dont retaliate on the Negatives you recieve. You are a Professional so you must Act like one. You can reply in plain view to any feedback recieved so your buyers can see your side of the story.