If the person OUGHT to know better - like some SEAL-team wannabe poser, who's always trying to impress people with his gun knowledge, but doesn't actually know shit - I wouldn't have too many sleepless nights over it.
However, to rip of someone who simply doesn't know - like a kid with his grandfather's guns, or a widow or someone like that, is just wrong, and I wouldn't do it, nor would I have respect for anyone who did so deliberately. Plus, I think in cases like that, if you actually try to help them, and point out the real value - they will likely STILL give you a really good deal, and you can feel good about yourself.
To me, it's ALMOST like finding a wallet with money in it, and a business card. You could certainly keep the money and throw the wallet away, and nobody woudl be the wiser (except you) - but when you do return a wallet, the person will often give you a reward - and even if not, their gratitude might get you out of a few years of purgatory (if you're catholic, that is
).