Hello, I’m Nathan, and I’ll be assisting you today. My livelihood depends on your positive feedback, so your satisfaction is my highest priority.
Sorry to hear you’re having trouble.
The fastest and easiest way to do this is to deregister / factory reset the kindle.
That can be done all in one process. Just go into the kindle menu, tools/settings, then look under Account, Advanced/About, or Help (depending on your model) and you should see a Factory Restore (restore defaults, factory reset), which will both take off your account info and delete all books. It will be like the kindle is brand new to the next person using it.