How to sell credits as products, using commerce module? The ideas is to allow users to buy credits through Paypal etc. and only using that credits, they can buy 'real' products. I appreciate your hints to set this up.
Have a look at the Commerce Userpoints* project. It allows to buy things using userpoints and there is also a feature that allows to buy userpoints that is based on a custom product type and rules, so fully customizable.

It most likely doesn't everything that you need, but it might be a start.

There is also Commerce credits and more similar projects, but I haven't tried them out, no idea what exactly they do and how well they work.

*Disclaimer: I am the maintainer of the Userpoints project and I was also involved in that project and still have some code lying around to re-work the UI and improve it.