In the case of the Purdeys & the Perazzi it's simply a moral issue as the widow didn't ask a person who we would expect to be informed as to fair market value.

In the case of the Colt Police Positive it is both a legal and a moral issue since the lady asked for help from someone we would expect to be informed.

That said, I have several times paid much more than the asking price because I knew the seller had no idea of market value of an item even though I was under no obligation to inform them.

I do like to sleep well at night & I do care about my reputation as an honest person as opposed to being an opportunist!