I was looking for something that I knew was in Video Production and Editing, which is a category under Cameras & Photo. However, the simplified page you get when you click on Cameras and Photo does not show that category as an option. I am pretty sure this is true for many other categories as well. As a seller you would have to be very careful not to list your item in a category that is not even accessible.
In the end I had to search for some similar item and pick the category from the page of that item.
Surely there must be a way to select from all sub-categories in a list?