WP automatic can post from any ebay subcategory beside main categories also
here is the correct method of posting from an ebay sub category
visit the ebay all categories page
once you opened the categories page now open any cateogry and copy the numeric id that appears in the url
in this case the id is 20096
now go to the campaign settings and set the custom category with the id as follow
update your campaign and you are done. now the plugin will search this category only.