Is there more than one eBay account on that machine?
I have seven accounts and that 'accusation' turns up more than half the time. It's really a protection for the 'real' buyer, preventing random access to the 'real' buyer's financial information.
Go to the top left hand corner where it says Hi (your personal name). You may have to drop down the menu, sign out from the 'wrong' account and back in with the correct one.