Hi.  As I don’t work for Sonos I cannot comment on the particular unit, but here are a few things to consider:

1. The Play:3 (there is no Sonos Three) was discontinued from sale 4 years ago.  So even if the box is unopened and the unit unused, it has been sitting in unknown storage conditions for at least 4 years.
2. It is extremely unlikely that an authorised Sonos retailer or reseller would be selling stock that old.  And if it is being sold through a site like eBay or Amazon Marketplace, this is not an authorised seller.  If you buy from an unauthorised seller, guarantees are invalidated.
3. Current Sonos speakers have some features that the Play:3 lacks
4. the Play:3 has the smallest amount of memory of any currently fully supported Sonos speaker / player.  It may well be fully supported  for many years yet, but almost certainly not as long as current products.

Personally, I would leave well alone, but you may be less risk-averse than me.

This thread may also be of interest:

Thank you very much for your detailed response. I should definitely request a live test should I pursue with this.

It may be that a Sonos employee will comment on that particular unit.  I don’t know if they would though.

The unit was manufactured in September of 2016.

Oh yeah I didn’t notice that.  Good point.

@ameyer .  Just for the record, the 1609 is not part of the serial number, but it is very useful info

This doesn’t feel right to me.  I would not touch it with a bargepole at any price.

Alright. Too fishy to be good. Will avoid and look for something better :)

Good call.  Although it would (I assume) require an increased budget, A Sonos One or One SL sounds very good, and a stereo par, IMO, sounds better than a Play:3.  

A pair of Play:1s also sounds great if you can pick these up used.

Will definitely look that way. I see Sonos as also official refurbished products (thanks to this convo I looked up more)!

Yes, let's just throw away perfectly fine products because maybe they will stop support one day... I run 4 play:3's without problems and they sound better than a One. I even took apart a few to build custom speakers and they still work. 

I don't get it... And Sonos is creating this vibe of throwing away good speakers to sell new ones, but that is already discussed many times before.