Many of the inquiries about not being able to purchase occur for popular products. Many customers are purchasing popular items at the same time, and placing the item in the cart does not guarantee stock, but other customers can also purchase the item at the same time. (All online shop platforms basically operate like this) Inventory will be secured by purchasing items in the order in which the payment is made first, so if inventory runs out during the purchase process, payment will not be possible and a notification will be displayed indicating that the purchase is not possible.