Knowledge is power and if you have done the research and know something the seller doesn't I see no reason to inform them. IMO the burden is on a seller to know what they are selling. That being said I have on occasion purposely bought something else off a seller that I knew I was getting a great deal from as a sort of "thank you" for allowing me to get a great deal.
I remember when I was quite a bit younger (maybe 15-16 years old or so) at a show I tried to inform an older dealer that he had made a mistake on a card and it was worth more, he told me to F*** off and got pissed, I learned my lesson that day it is best not to patronize the seller and to just buy the good deal and walk away.