uc_file module that allows selling files in Ubercart is definitely not perfect in terms of API, but it is a nice piece of functionality.
We’ve just finished creating one e-book store which has product-import feature.
Each product has multiple formats (pdf, epub, rtf) - some products can have pdf+epub, some can have just pdf, others have all three formats attached.

On product page, we wanted to show nice block like this:

where pdf and doc are just text for regular users, and download links for users that already bought this book.

All the code was created for Drupal 7 + Ubercart 7.x-3.x

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