Op; excellent subject that goes well beyond a gun show really, I've found the responses very interesting and telling on both ends of the spectrum thus far...thank you.
Information is power. As people, we exercise this fact every day of our lives almost unconsciously. However, every now and again we witness something that has us check our moral direction. Ask yourself; when you were younger, did you not see one of your buddies about to engage in something that was clearly not going to end well? Did you interrupt/intervene...or, did you pop corn, take a seat, and await the spectacle to unfold for the sheer entertainment factor?
In the situation the OP witnessed, I think it would be pertinent to prove the suspicion that the vendor is deliberately misleading a potential buyer (Perhaps the vendor is misinformed himself to give him the benefit of the doubt.

). A few pointed questions would clear the field..."Rarest of the rare, how so? I thought the Russians made 100,000's of these under extreme war time conditions?" or a simple question on mismatched parts/ referb stamps will resolve the question. Then; armed with verified information, you have to ask yourself do you share the truth of the matter with the potential young customer? Would it even be appreciated? The young couple may misinterpret your actions as trying to undermine their "find/purchase" and would therefore treat your "help" as intrusive. And; if they do, at what cost to yourself could there be to point them to the light in any regard? Walking away from a situation knowing you did the right thing can be very self satisfying, (I'm not even going to address the ridiculous social justice warrior nonsense) in the alternative, rolling around in a parking lot with a vendor or the male aspect of the young couple is a real and unintended consequence for "getting involved"...I suppose you should be prepared to deal with any and all outcomes whether you become involved or not.
In short, I think I know what I would do. I believe I'm a "moral" person, but I'm no different than the next guy when I say I'm complicated. On the turn of a particular, my actions would also most likely "turn".