Do you prevent people from copying your sold Mac App Store apps to another computer and running it there?
The Development UserGuide gives a hint for doing this.
How does this process work?
In MacOSlib there’s some code that you put in your apps open event. It will validate the receipt for you.
But an app bought from MAS is allowed to be run on up to 5 different computers.
Here’s a great article on the subject: http://www.tempel.org/RB/AppStoreGuide