Ah okay, that makes sense. And as fuinha is asking for a way to purchase my app outside of Google Play, I would asume that they are neither using a cracked version nor interested in doing so. Except for the fact that I now know why so few apps use In-App billing for pro versions and why many apps are not available from Amazon. But hey, gradle flavours and all (after some heavy refactoring) may make this rather painless. Apart from implementing IAB again.
So the answer stands firmly at maybe. I will look into it and hopefully update the thread when I am sure either way.