> If I own my content and can put it on any device I want, how can the seller be sure I’m only using one copy at a time?

They can't, which is the same as it is now. DRM is a farce and everything is on The Pirate Bay. Which is why restricting libraries like this is ridiculous -- anybody who doesn't care to follow the law doesn't need to get a copy from a library, they can just download it from a piracy site. The reason you go to the library instead is that you want to follow the law.