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).
That's the whole problem with Ebay and Paypal. They're not designed for the buyer. Only for the seller. Ebay charges the seller to sell and item. Paypal charges the seller for the price of the item sold. They make money off the seller, not really the buyer. Hence, they're always going to side with the seller first and foremost.
It's the same problem with the unemployment system in America. Companies pay into the Unemployment system, as insurance. The employee doesn't pay into it. So, when an employee is unlawfully fired, it's next to impossible to win, 'cos the state will almost always side with the Company. You could have evidence supporting your case and they'll still find a way to side with the person they collect the money from.
Capitalism at its finest.
Ebay and Paypal will never change, until people stop using their services. End of story.