It is quite popular feature request to notify on items that are out of stock at POS.
How would you prefer this to be implemented?
1. Do not show items that are out of stock at POS (make them unavailable for sale).
2. Get prompt message about those items when trying to add them to a ticket. In this case you still would be able to sell these items if you physically have them in stock (if you have the wrong record of your current stock, e.g. if you did not update it in time after supply of goods).