That's very odd. I thought "pre-approved payments" to eBay were usually refunds etc. that had sellers had agreed in the user agreement could be taken from their PayPal account.
Try using this link to see what you've been charged for: http://pages.ebay.co.uk/help/sell/invoice-display.html. If you've received any invoice from eBay this should show what it's for.
There are other possibilities. Have you ever taken out eBay's optional "square trade warranty" for anything you've bought? Or bought anything that involved taking out a subscription? It could even be that you have picked up some malware on your computer that tricked you into taking out some sort of subscription.
I presume you've logged in to your PayPal account to see whether there's any more information in the transaction records? Failing that I think you can only phone PP customer support for advice. You could also phone eBay customer support, but they're so hopeless you can't count on anything they say.
