> <snip> write something once and sell it a zillion times. Of course first you need the idea and the time to do it....

Unless you give your customer all rights to the app you built for him, there is notthing stopping you from selling a zillion copies of that app. I know a developer that started this way and is now selling a fraction of a zillion copies.

Starting from your own idea is more difficult. 1) It is difficult to come up with such a great idea while up to your ears with projects from customers. 2) When you have the idea and start programming you can't bill someone every month to pay for rent and pwoer and all that stuff unless you at the same time do projects for customers and then your great app is delayed and someone else wrote it and is selling a zillion copies.