Ben, that thought occurred to me, though I had and still have no a priori reason to doubt the seller's honesty. The two most likely scenarios seem to be:
1) the lens was not used for several years (the seller told me this) and a film built up on the contacts unbeknownst to the seller, or
2) the seller knew, but didn't know about the eraser trick.
I tend to believe the first case, because the seller is an active photographer but sold his M7 body some time ago, thus would have been unable to test the lens and shutter functions. But either way, I got a very nice lens, probably KEH EX+ or better, for less than half of KEH's bargain-grade price, and am very happy with that. And I already had a Pink Pearl (brand name) eraser, so I accrued no additional expense beyond a few wasted frames of film.