Well, since he calls it a folding knife I would take a bit of caution. What I mean he is obviously clueless about straights and probably wouldn't be a reputable dealer.
It's hard telling...it looks like it is a Dovo Best Quality, which I own two of myself and they are great razors. However, I've never seen one with white scales, am unfamiliar with that dealer, and am cautious when it is referred to as a knife. If however, it is an untampered Dovo Best Quality, I would say it's not a bad deal since this razor on SRD goes for about $10-$20 more.
I would be cautious though and besides, you could always go buy one from SRD and know that it is shave ready when you get it rather than having to send it out for honing after you buy it and having to pay for that as well.