What's the best way to implement a system to allow the end-user to download and buy documents? I need some advise on how to build a system to allow users to download and purchase pdf files. A purchase option is a one time only thing and no order history of login is required.

The end user just needs to see a list of documents, some of which can be purchased and then when the hit buy the can pay at a Payment Provider and download.

I don't need a shopping cart, order history, login or anything.

What is the best way to approach this with Drupal 7? I was thinking of maybe using the Entity API to create a download entity, would that make sense.
There's a module in progress that provides that for Drupal Commerce, here:
http://drupal.org/sandbox/recrit/1118266

You could use it as a starting point, or just for inspiration.