To make sure you have access to add items to the store and manage inventory, You will need to make sure to add the action Payables > Items for Sale to the role that users setting up the items for sale will be assigned.
This can be an existing role like Administrator or you could create a new role called Store Manager that you add the action to and then assign that role to just the users who will be adding items to the store
From Bills >Items for sale, you can add new items for sale by clicking the new button
no sale - just a link : Check this box if you are directing the agent to a third party site to order and pay for the item
item : Name of the item
price: price of the item
image: upload an image of the item
For this type of item you typically would not check any of the next 5 boxes