Assuming that it is the same stone this puzzles me too. The price comparison of other stones we stock in common are similar so I have only two thoughts - it's older stock and he paid less for it than we did and (probably definitly) I bought my inventory a few months ago when the dollar was low, he bought his when the euro was high, now with the currency's reversed his price is much less expensive - ours stays the same. Incidentally we don't even make our usual markup on these stones I doubt he is either.
All this shows is that in the exciting dynamic world of retailing vendors aresn't selling commodities. If you are comfortable with a vendor, and you like his prices, buy from him. He is also a guy who specializes in razors and I would guess for the members of this forum that's a reason to support him. If in this case the pricing is an accident of currency fluctuation I would buy from him NOW because the Yen is skyrocketing.
Long term I hope you look at our unconditional warretty with us even paying return shipping and our expertise in the sharpening arena. And of course we are local for people in the USA.
joel