To add products to listings, you must first edit listing type in WP-admin > listing types.
This field will show when adding / editing a listing and display the products created by that user in a multi-select dropdown.
To display these products in the listings, you must go to Single page > content and tabs > create the "Store" tab and choose the "Products" field
That should be it. Once you choose a product when adding or editing the listing, it should show on that listing under the "Store" tab
To allow your users to add products you must use a vendor plugin. At the moment we recommend the plugin Product Vendors