Is is possible you made a purchase using a different eBay account or as a guest?
If you are certain that you made no such purchase, and there is no indication of a purchase in your Purchase History in your eBay account(s), then contact your bank for details of the transaction or to dispute it and get a new debit card issued.
eBay can only help if your own account is used to make a purchase without your authorization; if someone obtains your bank card number and uses it to make a purchase with another account, you need to contact your bank.
