The seller sounds pretty knowledgeable about how things go on
ebay. Nuff said on that score, I think. However, I think he is crediting the ribbed case with more umph than it is worth. 2015 Scott for this type without ribbed frame is $375 and with the ribbed frame it's $575. If the mica were intact I would agree that this could very well exceed $1000, particularly to a knowledgeable specialist. But that same specialist will hold out for a sound example at double cat rather than pay even half or 1/3 cat for one with such a serious face problem. That's usually how it goes in the stamp world.
However, before I considered my homework complete on an item like this I would check the realizations at Bowers and Ruddy as well as Heritage to see if there is a pattern or trend that suggests something else. As I said earlier, numismatists are on top of these things more than philatelists are. You may have better luck getting someone to ante up for the ribbed casing if you find out something about that, why it was done but only by a few, or how they fared in circulation. Heritage can give you loads of the kind of detail I am referring to. But you may have to use the numismatic cat identifiers for searches at those web sites. The main cat being offered on
ebay is of numismatic origin, and may or may not cross reference to the Scott Specialized (though I doubt it).
The specialist doesn't need you to give the background on the ribbed case. S/he needs you NOT to say anything, because that informs and raises the prospect of competition from new places. Information is king.