I sold a house with a bunch of smart home stuff last year, but it was all commercial stuff, nothing like a HomeAssistant system. The buyer wanted it and I didn't (and it would have been a hassle to remove), so it was a win/win for me. I just transferred all the accounts and/or changed the emails and passwords, gave them the credentials, and walked away. I do think that this is very situation dependent and there's risk of getting into weird situations where the buyer wants support or something. But I think that as long as the buyer reasonably knows what they're getting into, and you're transparent about what you're leaving, you are under no obligation (either legal or moral) to provide post-sales support...