Smart objects are tied to homeowners in ways a regular thermostat or lighting fixture are not, but they are still considered fixtures. Before selling or buying a property, you need to know what smart objects will stay with the home. Since many Smart objects will stay with the home, you need to be clear on how and when their ownership and access will switch hands.
If selling you should remove all personal data and reset the devices. You will need to go through each system that stays with the property to see if it represents a privacy issue and what to do to clear the system. When systems are going to stay with the property you may need to transfer ownership and create new passwords.
New owners should open new accounts asap and install any critical home technology updates. You may want get a list of all Smart devices from the Seller and the account information with each system. Reset the devices and verify privacy settings. And always update passwords!