If you have a store, listing items in your store is easy.

in the first part of your listing page (at the top) you will see a "List In" dropdown box with the following options:

• Site
• Store
• Both

Choose where to list your item by selecting one of the options.

Site - will only list items as normal and will be viewable under the main category you choose.
Store - will only list items in your store and will be viewable in your store under the main category you choose.
Both -  will list items as normal and in your store and will be viewable under the main category you choose.

You can change this at any time so long the item is active.