I see a lot of people sell their Kindles with books loaded. They do advise the buyer that the books are DRM and if they delete them from the reader then there's no way of getting them back since the buyer will not have access to the seller archive files. They usually list the genre, book titles, and authors.
I see nothing illegal about it because I had problems with Amazon when I could not register 1 of 2 new Kindles bought by 2 friends to my account; I returned my 1st K1 for a replacement and my K2 twice for a replacement. Amazon told me the reason that I could not register the 2nd kindle is because that would make it 7 licenses; I was like hello, I only own 1 Kindle and that's a K2 (my friends 2 K2 would have made it just 3 Kindles), they removed the licenses of the returned Kindles but not the K1 that I kept until I sold it to buy the K2 in February. So if I knew that before, I could have left all my books on the K1 since it's eating up one of my licenses and I don't even know the new owner.

Of course, I decided not to register both of my 2 friends Kindle on my account and had Amazon remove the 1 that I was able to register since I had yet to transfer anything to that Kindle. But this DRM complications is a big mistake because you find ways around them because my 2 friends are reading some of my Kindle books on their Kindle,

it's just now I decided that Amazon would not be in the loop!