A listing contains the products you have for sale.
Without creating a listing your users won't see your products (even though you've created them on the Products page).
Creating a listing is easy, simply go to the IAPHUB dashboard and click on the Listings page in the menu.
The listings are versioned, click on the "Edit" button of the last version to edit it.
Click on the Add product link to add a product.
If a product doesn't appear it is because you did not create it on the Products page of the IAPHUB dashboard.
When you add a product that belongs to a subscription group, you must return all the products of the group.
Also do not forget to save after you're done editing the listing.
Copy previous listing
When you edit a new listing it is by default empty.
An option is available to load the configuration of your previous listing.
You just have to click on the 'three dots' icon (next to the save button) and click on Load config of previous version, a modal should appear.
Saving a listing isn't enough to make it available to your users, you must publish it.
If you already had a listing published, an option will offer you to unpublish your previous listing. Unpublishing a listing is necessary to allow the users that already loaded a previous listing to load the new one.
You may want to keep the previous listing published in the case you want to publish your changes for new users only.
If you're wondering what's the Smart Listing option, I would recommend take taking a look here.