Hi
You need to create sections first and then select them when you are listing, rather than writing it in manually.
To create your sections you need to click on "your account", then "info and appearance", and then choose the middle tab which is "sections", here you can create up to 10 sections. You can then put all of your existing items into a section, and when you list in the future you will be able to put items straight into sections as they will all appear in the dropdown box.
Hope that helps, shout if it doesn't make sense!
Jo :D