Can't start a new listing ? - When I try it just brings up an old (sold) previous listing. And the category doesn't exist in my products, so that is no help !

Kia ora kmorrissey, have you tried the troubleshooting tips? Clearing cookies and cache often helps. It sounds like you're trying to create a new product in My Products, if you pop some screenshots or tell us what you're trying to list, there are lots of MP users on Community, who may be able to help you.

Yes Michelle is right here. Usually if an old listing comes up it points to a stuck cached page. So clearing that should sort it. 

My Products has all the same categories as the standard website, so not sure what you mean there. 

Although some special items like classified ads across pets, vehicles, or property, can't be done through MP