Paying users only if they download and paid from the market. How?

Suppose that with my app, users are payed (with true money, don't care 'how', in this question) when they conquer prefixed goals.

Obviously I want to pay them only if the app is not hacked! Matching, for example, their phone account 开发者_运维知识库with the account used in Google checkout.

Which approach will you use?

Take a look at Licensing Your Applications on the Android dev site. It uses the user's main Google account (as used for the Market) in order to do license checks, and you can also restrict the use of your app to a specific device.