Such a system is already in place, seperated into 2 parts:
The first is called garage (or app manager on your device).
The second is called "paypal".
IMO it would be better to get the garage admins to implement some sort of automatic/integrated paypal scheme.

But then again, a different approach wont harm anyone.
However, Google already knows about my mails, search habbits and my exact location and movement profile (if I activate the geo location stuff anyway). No need to give them even more infos about me.
Hence, I hope the new approach would be a wrapper around existing apt repos so you do not HAVE to use it if you want a certain app.