Jimmy,
If this was an easy one you wouldn't have asked us. I do see the "damned if you do, damned if you don't" component to the situation and it's not clear cut to me.

Early in life I learned butting into other people's business without being asked is rarely the right thing to do unless something serious is involved. And a superposed doesn't make it as serious in my book.

The buyer knew enough about the gun to recognize the price was too good to be true, and jumped on it anyway. A guy with 2 G's to drop on impulse without checking a thing?

If it was a working young guy buying a broken down pump for more than Wally World sells them new, I'd be more inclined to butinski.