Implementing In App purchases in Android?

This has changed as of today! There is now an example on the Android Developer site here: https://developer.android.com/google/play/billing/billing_overview

You can create a premium key application, that will have a key. How you expose that it's your deal ( or you can just check if PremiumKey activity exist ). From the main app you just check for your key and if it's exist enable premium option ) If you're talking about buying OTHER apps from yours - build your list with market url pointed to the other apps ( market:// )