I'm afraid that eBay won't be able to help you. The buyer was supposed to pay through eBay, but instead you somehow agreed to let them pay you (or not pay you!) through PayPal. It's a very well known fraud which only happens when sellers don't follow eBay's procedures correctly.

You have been the victim of a crime. Online fraud is now reportable to Report Fraud,  https://www.reportfraud.police.uk/  This has replaced the former Action Fraud.