Too Long; Didn't Read
Developers should put up their apps for sale when they have a new idea they want to work on. The maintenance of an old app will still take away a lot of your free time. The old app deteriorates in the process as well, and you get fewer and fewer resources as you go by this pattern. When you reach the point where the whole app needs to be re-done from scratch, is when you’ll know that it’s too late to sell it. The maintenance is getting harder and harder for the app to remain relevant as time goes by.