Ah yes, I’ve just been through that; we moved house a month ago. The new owner wasn’t interested in Hue lightbulbs or in our Honeywell Evohome heating system, and they were worth enough that I decided to remove them both.
The Hue stuff was a bit more work than you might expect, because I had removed all the old light switches and replaced them with blanking plates to avoid confusion. Fortunately, I still had the old switches and I had labelled all the wires in the more complex situations with double switches at the top and bottom of stairs, etc.
For the heating, I bought a batch of very ordinary TRV heads to replace the smart Evohome ones, and a standard Honeywell timer and thermostats which I had to wire in. This was much easier because I’d done it before and, again, because my wiring was much more organised and better-labelled than what I originally had to deal with!
Perhaps there’ll be a new ‘dumbing-down’ service that could be offered as part of a sale: making your home less smart so the new owners can decide hoe much they want to smarten it in their own way 🙂