If you've ever bought or sold a used phone, chances are you did it through Swappa. Swappa is one of the better sites around for safely and easily swapping out your old hardware for something new, and the site recently added support for doing this with smart home devices.
Listed on Swappa as "Home Tech", you can now buy and sell Google Home and Echo devices, Sonos speakers, smart vacums from iRobot and Neato, as well as Nest's library of security cameras and thermostats. At the time of publishing this article, you can only purchase the original Google Home, 2nd generation Echo Dot, Nest Thermostat E, Sonos Play 1, and Nest Cam Outdoor. However, as time goes on and more people start listing their gadgets, the availability of these things will increase.
You can currently pick up a Google Home in good condition for as little as $85, the Echo Dot is selling for just $35, and the Nest Cam Outodor that usually sells for $199 is going for $125.
As long as you don't mind buying something that's (gently) used, this might be the way to go if you've got someone with smart home gadgets on their Christmas wishlist.
