I've bought a couple of things from Kevin. For example, I have a 35mm Summicron IV because 1] threads on RFF at the time rated it highly, and 2] Kevin had one with a cracked focusing tab, glued together by the previous owner, with perfect glass for about $200 less than the least expensive going price at that time on the usual auction site, or any other source I could find. I later got the focusing tab replaced by Gerry Smith, and my total outlay was less than the price I've seen almost anywhere. Kevin charges more for cosmetic condition than glass condition, maybe because he sells a lot to collectors. He also sells gear I haven't seen anywhere else.
He's expensive, but I regard a purchase from him as a kind of insurance. Based on my experiences, I get what I thought I was buying, in the condition he described, or better to my eye. That is not true of some other vendors; I won't buy from the usual auction site unless I know the vendor.