Telling your third customer in a row that you are sold out of popular item is more than frustrating for a shop manager. Losing a sale not only effects your numbers, but it means that you might lose a customer, or their loyalty which can be worse.
When you lose a sale to a first time customer, they will most likely move on and look somewhere else. If you lose a sale to a loyal customer, they might look somewhere else as well, and that ‘somewhere else‘ will most likely be at one of your competitors. Each time they come up empty handed, you lose a little bit more of their loyalty. If this happens too much you can lose them altogether. If that happens often, you will not be an business for very much longer.