Not op: Can you add some unique tracker/signature per purchase so you can tell what copy was used to upload. Probably pointless with regard to the stolen cc comment/new accounts.

Sure :) The biggest barrier is micro-payments. If I could charge as a streaming service, but without having to go through a major platform (I don't want Spotify-for-books), then that would be perfect. I actually pitched this to GrantForTheWeb using their Web Monetization toolkit.