It's not that easy, say for example if a over excited user clicks the purchase button 20 times, ending up in 20 simultaneous orders. What is the best way to handle it? Should the user get 20 orders shipped to him, or only one? And should the user get a message that says "order already processed" or "order processed successfully"? You can have a system that works correctly 99% of the time. The devil is in the 1% of time when things go wrong.