Couldn't they just go to Ringo and ask him if he ever played them and when and where and so on. Even if it was not at a public performance, they can at least get the details on whether or not he played on them just by himself or even with some friends jammin (famous friends). It would help clarify and keep them out of ambiguity territory.
I would think that Ringo would give them a conversation and that this seller (music store in London) would be able to get in touch with Ringo. Seems all you would have to do is prove you are the one that bought this very kit from Ringo's auction.
One thing for sure - to snag that up for $17000 then turn right around and try to get $60K for it. That is QUITE speculative. If it did not sell for more in the first auction in the first place, why would it sell for more on Reverb?
It would indeed be a hard sell now or any time in the future, as while this may have been Ringo's at one time, it is not THE Ringo Premier kit, so any discussion of that would be misleading and of no special value, since, again it is not THE Ringo Premier kit.....and never will be.