When creating your Retail Products, you have the option to link these products to your Inventory. If a Retail Product is linked, whenever you make a sale, this will adjust your Inventory/Stock count for that item.
To do this, you'll firstly need to have created the product in your Inventory.
Once you've done that, head to:
> Settings
> Retail Products
> '+ New Product'
> Enter the Name
> Then below you'll see Tracked Inventory - clicking into the field will bring up the first 15 items, but if you start to write the name of the item, this should start to appear
> Select the relevant item from the list to link this back to the Inventory item
> Continue to set up the Retail Product, by entering in the Price Rules
> Save