ASP.NET，C＃：如何处理购物车问题

I am developing an e-commerce project, in this I have one crucial product that have only one ProductQuantity,and that product have quantity 1 so two person can not buy that product. at the time of AddToCart I have reduced the quantity from the database. I have test this with 10 machine and at the same time all 10 machine click on the AddToCart Button,but 3 machine is able to add the same product to own cart, which is wrong, only one machine(person) can add that product.I am developing an e-commerce project, in this