Undoubtedly a good deal, but I have a bit of a problem with it.
If a dealer misidentifies or misprices a gun, I am willing to take it at his mistakenly determined price because he presumably has the resources and experience to price it appropriately. If a private party offers me a gun at way less than it is worth, I will correct his misunderstanding if I don't want it, and pay him something closer to what the gun is really worth if I do.
If I wanted that gun and got that offer, I think I would have given him $200-250 for it. I'd still be making a decent deal, and he wouldn't be getting hosed.
In short, I don't feel the need to protect someone from his own carelessness, but I really don't want to exploit another's inexperience or ignorance.