I was going to make a very sympathetic reply because, as you correctly say, how can private sellers now be treated as responsible for the delivery of their items? eBay no longr gives us the choice of delivery service, and eBay accepts responsibility for refunding buyers for items lost or damaged in delivery. If you dispatched it within your dispatch window, have you tried contacting customer support to ask for it to be removed?
However, the above applies to private sellers. You are currently listing around 700 items for sale, many or most brand new. Buying to resell on eBay is a business activity and it certainly looks as though, knowingly or unknowingly, you may be breaking the rules by incorrectly presenting yourself as a private seller.
By not registering as a business seller, which is what you appear to be, you would be avoiding payment of eBay's selling fees, denyng your buyers their right to return items and breaking the law. I think you should be more concerned to look into this situation than about a single, and I agree unfair neg.
Q~~_7.JPG "plpmr"){kind=avatar}
