Such an Extension would likely be declined. Please let me caveat this with this is not definitive, as it would depend on the implementation, but I cannot think of a simple suggestion that would be accepted.
The best suggestion I can make today (which is not all that good), would be leverage the Amazon Gear extension, for Amazon saleable items, and have items for sale through Amazon.
Extension monetisation is something that is happening. This space is likely to evolve quite quickly, so hopefully more information will be available in the near future.