another opportunist's scam?  

They look like Herman Miller to me
but who can be sure without actually seeing them in person?
A vintage chair with vintage fabric probably had a paper label on the bottom of the seat, but this 'lovely' fabric sure looks like they were redone sometime in the 1960's or 1970's.
If they're real $2500 still seems like alot of dough; $416.00 each.

The chairs appear to be the same
as those in the black & white photos seller cites.
Seller also has flawless feedback.
Barry- a single dining chair that needs to be reupholstered may not be a steal at $416, but a set of six makes them considerably more desirable. You could spend years gathering together six individual (cheaper) chairs.

Legit
There is nothing nefarious about the ebay listing. Those are Herman Miller chairs designed by Nelson, it was model 4671. Obviously, they have been reupholstered... and need to be again. While you don't see them everyday, I don't think they are all that "rare" or terribly "important" for that matter. They were probably pretty innovative when they were first designed in the late 1940s what with the slender aluminum frames and all. Like a number of the early designs from the Nelson office, they are a modern approach to more traditional forms rather than a reinvention of the wheel. That there were a great many generic imitation of the form in the 1950s & 60s speaks well of the design I think.
There is a nice set of these currently on Wright Now.
http://www.wright-now.com/shop/chairs/DD/none/HTSP/HO34

.
I think those (on Wright) are gorgeous, but again I'm having to make an effort to appreciate them apart from every horrible '70s aluminum imitation. Seeing these is like looking at a Platonic original, but what I know best is the shadow copies mass produced everywhere.

I have always loved these...
I have always loved these chairs. They seem extremely functional and comfortable.
I agree, these are not extremely desirable to the collecting public as far as I know. I think these are grossly overpriced, especially considered that the original upholstery has been removed already. Plus the fact that reupholstering these correctly will not be cheap.

They look like simple box cushions
Why would reupholstery be unusually expensive?

Not unusually expensive....
Not unusually expensive. These look easy enough to handle ... Just expensive is comparison to buying a set that doesn't need it.

You're right, WHC, but it still
seems to be expensive, but I guess it isn't.....but when you figure in the reupholstery, it's still darn expensive.
I love my 4 EC127 upholstered Eames chair with the metal legs; upholstery is royal blue and they're just what I wanted!