It just happened to me today too. I've been a member since 2001 and this is the first time something like this has ever happened to me. I logged in today and received a message from another fellow Ebayer telling me my account is being abused. My account was selling a phantom bicycle that did not belong to me. I immediately contacted Ebay, had the listing removed and changed all my passwords on Ebay, my linked email account as well as my paypal account. It's so scary. And I think I know the reason how this happened too.
A few days ago I has listed a want it now ad and responded to a response from an anonymous ebayer. I should have known better. Needless to say, I also removed this want it now as well. Keeping my finger crossed that everything is in the clear now.