Many, if not all, of your items are probably items that you buy from vendors and sell to customers and store in inventory.
Note: Don't feel pressured to inventory every item
When you mark the I Inventory This Item selection, you're indicating that you want to track the item's on-hand quantity and value.
If you don't need to track an item's on-hand quantity and value, you don't need to mark the I Inventory This Item selection.