ThanksYes, it really is that simple!
Resetting it to factory defaults should remove all content and all your personal info and settings. It should also remove it from your Amazon account but it wouldn't hurt to check that it has. Luckily it won't remove any firmware updates that have been installed since you bought it.
After that the new owner just needs to follow the set up procedure and login to their own Amazon account on the device and it will then be attached to their account.