I've only ever left negative feedback once and I have bought a LOT on eBay. I'd bought quite a number of items from this seller and one item was missing when the package arrived. I immediately contacted them to let them know (nicely, as it could have been an oversight with such a large order) and I heard nothing back. I contacted them several more times and still heard nothing back. After 4 weeks of harassing them, I left them a negative for their efforts. 

I have since bought from this seller again and had a few large orders. A few months ago a package arrived missing several items. I contacted them straight away and they replied within minutes that they had somehow been left out but had been packed with my next order. 2 weeks later, my parcel arrived and sure enough, my missing items were there. I hadn't bought from them for a few years and they've really picked their game up as far as communication goes now.

New buyers should have to pass a test before they can buy, to prove they have read and learned the basics. That would never happen though!