will a counterfitter also counterfit the manual, box and bag?
Yes. Check out the pictures in these threads:
http://www.bladeforums.com/forums/showthread.php?t=643584
http://www.bladeforums.com/forums/showthread.php?t=715621
So far, the most obvious tell of the fakes is that the butterfly logo on all of them are facing the wrong direction.
It's too bad the picture is so blurry. To my eye it looks legit. I would be that the 51 (and 32) would be pretty difficult to counterfeit convincingly. The clip and liners would be the most obvious problems. They're titanium, electro-chemically anodized blue (maybe a hint of purple), and the liners are jeweled. You can't color steel like that, and a magnet would be an obvious giveaway. The only other possibility would be aluminum, and the clip, if it were made from aluminum, wouldn't have even close to the strength of titanium, and very little spring-back. Aluminum, even the best quality alloys, would be pretty easy to bend/break at that minimal thickness.
That, combined with the fact that he has the receipt (what was the original price, btw?) makes me believe that it's legit. $100 for a near-mint 51 is a steal. Even if it were well-used it'd be a good price. Caveat Emptor about the receipt though: If it's a California retailer (you being in Sacramento), some suspicion arises there. That knife would be illegal to sell in CA; many retailers outside of CA won't even sell balisongs to CA addresses. If it's from a shop in CA, I would wonder how they got the knife. Benchmade probably wouldn't have sold it to them, or if they did, the dealer wouldn't sell to anybody who wasn't active duty military or a law enforcement officer (with all the paperwork). However, if it is indeed a counterfeit, you can find those at swap meets, flea markets, gun shows, smoke shops, all over the place, regardless of legality.
If at all possible, in addition to the original price, see if you can find the dealer's name on the receipt as well (and let us know).
there are benchmade 51 counterfeits, very well made counterfeits. I encountered them in China while traveling. Have some photos if anybody wants to see.
Yes please. I don't think we (the online balisong community) have seen any fake 51s yet. It would be an appreciated service to us if we can spread the word on any possible counterfeits before anybody gets burned.