"If a buyer wants a warranty for it (the Zstage), is that something that can be reinstated (if seller does not have the warranty), or is it now permanently void?"
Here is a link for Decware warranty info,
https://www.decware.com/newsite/service.html, within which the paragraph 'PURCHASING WARRANTIES', wherein it states:
"If you have acquired a Decware amplifier somewhere on the used market you can send it to us for re-certification where we will make any necessary repairs and charge you for them. At that point, if you wish to purchase a warranty for it, the same costs apply (as described above)[higher of 10% or $100]. You will also be required at that point to purchase a new factory tube compliment that we test in your now certified and warrantied Decware amp."
On that info page there are separate paragraphs for transfer of warranty and purchase of warranty, so I presume the latter fits the case for your potential buyer.