Why is it complicated? Factory reset is simple, arguably easier than resetting a phone/tablet for resale. It wipes all your account info and settings from the device. 

As for your Sonos account that just remains dormant. 

Yes agree, but I thought if you sell on to unknown buyer,   when they set it up does the serial number for that item get blocked for the new person as it will show still on my account, bit like apple does.

I’m not sure how much simpler the process could become without having lots of users accidentally stumble over a dedicated hardware button or one click option to delete the account and factory reset everything in the system.

If you will not have any SONOS units remain, you could ask SONOS Support to delete your account. This process cannot be too easy, otherwise there would be a lot of pranks.

Ok thank you,  yes this is the only item I own by Sonos, so your suggestion I like, will get my account deleted by support.

The device would simply disappear from your account when it is registered to the new owner's. 

You can also fake selling it by using a throw-away e-mail address. Reset and register to the fake address with fake user info to clear it out of your existing Sonos account, then reset again to pass to the actual buyer.

That avoids having to wait on the buyer to register the device.

That’s a good idea of yours, I’m selling it to a electronics shop, and they always like you to clear down anything linked to the items.

when selling Apple items you need to take off your cloud details, but Sonos not having this kind of info, it just means if the item gets stolen, they can just get rid of it without being traced.