I wouldn't bother trying to report it to eBay, blues8447 - they're not bothered and nothing will be done.

Items turning up out of the blue usually mean that the seller (usually a Chines-based business with a fake UK address) has randomly chosen your address for returns: the item gets delivered (to you) and once registered on the system a refund is sent to the buyer. It's not right but eBay don't verify return addresses (they could but don't) and it means the seller doesn't have to pay to have the item returned to China.

It does seem odd that you've received an empty envelope but it could be a buyer, knowing the seller is in China and will use a random return address, is scamming a seller: raise a return, get a label which will almost definitely be a random address, send an empty envelope and get a refund. And as an aside even if the return address wasn't fake, the system registering the return as delivered would trigger a refund whether the actual item was returned or not - it's the delivery of the label that counts, not the delivery of the item.

"CityhealthUK" may be the company name but the eBay username is probably different which is why you can't find it. Are the any other clues on the label - order number, item number, something like that? They may help track the actual username.

I doubt very much that your PayPal account has been hacked. Their security is, er, slightly better than eBay's. Every time I log in I get an email telling me I've logged in from a new device because the ISP server isn't near my location. Keep an eye on it by all means but I think your PayPal is safe. As for your bank account I'm not so sure - you presumably gave those details to eBay and they'll have been passed on to Adyen. That's the one I'd be more worried about.