It's almost certainly against the Terms of Service agreement. Presumably, you click-through agreed to a ToS for each one, and you have no right to transfer that agreement to someone else. The agreement is obviously tied to the human who "read" it, not whoever happens to control the account credentials.

Not that the ToS for your light bulb dimmer matters much in practice, but technically it's probably forbidden.

One might be able to work around that by creating a corporation, 742 Evergreen Terrace Management Company, in some state where it is cheap to incorporate, and having the corporation own all the accounts related to the house.

When you sell the house, transfer ownership of 742 Evergreen Terrace Management Company to the new homeowner.

That's not entirely outrageous. There are even existing Corporate and Trust structures in use for ownership transfer needs (mailbox keys and similar shared infrastructure that maybe can't be directly deeded/titled) in deed-restricted communities (Home Owners Association) and condo buildings (Condo Owners Association). A "smart neighborhood" could work to make sure that smart home functionality transfers smoothly between home owners.

Yes, I suppose that does seem like an interesting solution. But it seems like more trouble to go through than it's worth. Not to mention that if the new owner is sufficiently paranoid, then maybe they wouldn't trust that changing the password is sufficient to remove your access. So it would be all for naught.