Making a store is very easy actually. the actual hard part is finding good things to put in one.
All you have to do is have a points field in your users table (or wherever). Then, on whatever page you have your apps on, check if they already own the app and if they don't display the the form to but the app and subtract the amount of points from the user's points as much as the app costs.
It's my job to help the next generation, and set a good example for them. And for that I'll gladly lay down my life.