Hello and thank you for the question.
In order to wipe the Kindle clean, you will do the following:
Go to----->Home > Menu > Settings > Menu > Reset to factory defaults. This will clear all titles, return their licenses to the account, depersonalize the device and de-register from the account.
If you need further assistance let me know. Please do not forget to click at least a 3 STAR RATING, at the top of the page for my efforts. You can follow up at any time if needed.