The method is this:
On your Kindle device, to switch it to Mark's account and thus get acces to his books, you go to
Register (for your Kindle)
and choose Deregister.
When you choose that, your Kindle is un-linked from your account, placed in a neutral status, may perhaps be erased of your books also, and is, after a bit, open to being
Registered... to Mark's Amazon account, with his account name and his account's password.
Then your Kindle will, after that, have access to Mark's books, and can order books and charge them to his account, etc. Just as if it were his Kindle.
Is that a good solution for you?