I would say that you've been hacked. Good move changing your password. I would contact ebay and let them know that you've been hacked and someone has bought the item the hacker advertised. At the moment, you can only use the online chat option to contact them. Click on help at the top or bottom of any eBay page and scroll to the bottom. The chat option is only available during business hours, so no point trying to contact them now.
You will probably get a negative feedback when the buyer doesn't get what they paid for. Get in touch with eBay again, explain what happened, and they should remove it. If you are able to see who the buyer is, maybe send them a message and explain to them that you didn't list the item for sale. Hopefully they can put in a claim for a refund if they've paid.
