Use the well-known marketplaces for selling used equipment. There’ll be plenty of takers.

Trading is not permitted here.

According to your profile, despite suffering for 3 years you apparently only posted about such troubles 4 months ago. Suggestions about your network were made by a few of the regulars, but there doesn’t appear to have been any further comment. Nor was there any sign that Sonos Support had been contacted for advice. 

This is usually caused by solvable network issues, but since you have not been able to resolve them in the three years, you should investigate one of the online markets, such as Ebay. SONOS units usually sell easily for very fair prices. Be sure to Factory Reset any units that you ship out.

Or, you could work with SONOS support. They have access to helpful diagnostic data.

This is what you ALWAYS say… I already know this scenario ;-) 
» This is usually caused by solvable network issues

My network is good and everything works fine for having a UniFi system with UDM Pro on racks of professional items but only SONOS gives us problems and this for about 5 years.

I kept the system because WHEN it works, it sounds fantastic and I love it but you should ask for a better engineer to organize the network technical part.

Such a pity to have such a GREAT product from the sound point of view and destroy it with a wrong network engineer.

Exactly what is a Sonos network engineer going to do to fix problems with YOUR network?  I’ve been helping people for 15 years on this forum, and I can count on one hand the number of times problems were to blame on Sonos and not on the user’s network. Even those who \insisted their network was fine discovered it was anything but fine when told what to do.  

So I ask, did you follow the advice of the people here, or did you stubbornly refuse to believe it was your network?

Although there’s now a dedicated Sonos support operation for professional users, the system was -- and largely still is -- primarily targeted at home usage. It relies on a flat, open subnet with no impediment to the flow of traffic. Professional networks frequently need quite a bit of tweaking and cosseting to make them sufficiently open for all the traffic types Sonos uses.

Admittedly, in a wired/SonosNet configuration, Sonos don’t make life particularly easy by continuing to use a modified form of a pretty old STP standard. That said (a) in home networks (once again) this is rarely an issue, and (b) SonosNet appears to be gradually heading for legacy status anyway as more and more users adopt powerful WiFi meshes. 

Thank you :-)
What do you need to know from my network settings, knowing I am not en engineer ;-) 

» Sonos network engineer going to do to fix problems with YOUR network?

You have professional, enterprise-class network equipment. Surely someone there must understand it? 

I see

>> Surely someone there must understand it? 
>> What do you need to know from my network settings, knowing I am not en engineer ;-) 

This thread has devolved into a bunch of random » quotes and emoticons.  I suggest we leave it be, so it falls off the first page. 

Lots of talkings but no solutions…

this is Sonos politic I believe.