100000 users concurrent to buy the same product in ASP.NET

I am run into trouble.Let me say,There is a product table,The structure is:
ProductID ProductNumber ProductName Quantity 1 A346457577 AAA 2 My requirements:
Assume that per user just can buy one.
In my sytem When 100000 users concurrent submit the order at the same time ,the total quantity
is far more than 2. How to avoid this? queue? memcache? I have no ideas.Tks!