If Amazon really wanted to twist the knife, they would look into making the first-sale doctrine apply to ebooks. They might have to limit it to Kindle-to-Kindle

If Amazon really wanted to twist the knife, they would look into making the first-sale doctrine apply to ebooks. They might have to limit it to Kindle-to-Kindle sales at first, but Amazon would be able to take a cut of every book transfer in perpetuity. My guess is that it would drive authors into services like Kindle Unlimited, since they would be able to create a long-term income stream unlike physical books or non-transferrable ebooks.