Commerce File is an outstanding and mature module that allows you to sell downloads. It uses Drupal 7's private file system and provides a field that can be added to a product for the downloadable item. If you're interested in Commerce File, you may also be interested in the Commerce Downloads distribution, which comes with Commerce File completely preconfigured for use.
Here's how you set up to sell downloads with Drupal Commerce:
* Download and enable Commerce File.
* Make sure that your private files system is properly configured (admin/config/media/file-system).
* Add a field of type "Commerce File" to the product type you want to use.
* Create products and display nodes as you need.
* Let people buy!
The default rules provided with Commerce File grant licenses to the file when the order gets to the "Completed" stage. Your store may not be set up to automatically complete orders; you can change this with rules based on your payment strategy.
And don't forget to read the Commerce File documentation page, which provides more information.