That's the trouble with PayPal. You the seller have to prove you shipped something, otherise PayPal will always find in the buyers favour and you will lose the funds. It's also in PayPal (eBay's) interest for the dispute resolution process to take as long as possible because during that time they keep your money and make interest out if it!
I had to chase the buyer who made a complaint about me for non-delivery and beg them to remove the complaint (after they had got the goods).
