eBay categories identify a product’s main characteristics and group similar products together.

Select a Main Category
Select your main category for your product.

Select a Sub-Category
Select the sub-category in which you would like your product to be listed. Then, keep choosing sub-categories (of the sub-categories) until you have reached a primary category (when there are no further sub-categories from which to choose). This is necessary for a proper listing on eBay.

You will know you have selected a primary category when no categories appear in the subsequent select boxes on the screen.

More help on eBay is available here.