I had this happen to me so I emailed customer service. They immediately changed it back for me, and they also sent me the following message, so that if it happens again I can change it back myself. I have attached a copy so that you can give it a go yourself, and if don't succeed, just email ebay and ask them to change it back to original selling template.

"Thank you for contacting eBay about the new listing tool. don't worry let me help you sort this out.

I have immediately switch back your listing to our advance listing tool. You may now then continue with your listing.

Here's how I did it:

-In the 'Tell us what you're selling' page, enter 'Car' or any keyword for a Motors listing.

-Click 'Get Started'.

-Scroll down and select  "Continue without selecting a product".

-A pop up will appear advising that the category isn't available in our new tool yet.

-Click "Go to classic tool".

This will serve as your future reference, if in case you might need it. I trust this information is helpful. If any other questions arise, please don't hesitate to let us know.  We're pleased to help in any way we can."