Perhaps just ask the buyer to kindly factory reset them for you (they most likely will want to do that anyway, particularly if you perhaps change your Sonos account password aswell) and when they transfer the devices to their own (or anyone else’s) account during setup, they will automatically disappear from your account.

Hope that assists, but if you’re still concerned, then perhaps see if Sonos Support can assist you further. They can be contacted via this LINK and maybe see what they suggest.