I am one of the guys you guys hate. I always offer what I think is fair prices for machines that are under priced. My email is typically something like this: "If your machine isn't already spoken for I would be willing to pay between $ ----- and $ ----- depending on overall condition once I look at it in person". "But I will pay $ ----- regardless of what I see when I look at it in person based on what I can see in your pictures".
Me paying "fair" prices for machines drives my wife totally crazy because she keeps saying "you could have bought it for less money why do you do that". But doing that has gotten me so many more machines I can't even begin to add them all up. Paying a "fair price" for 5 machines by giving the seller a few hundred extra a few years ago got me into the warehouse deal which was a total of over 80 pinball machines plus arcade machines & jukeboxes as well as a bunch of parts. People remember buyers that are "fair" with them and refer other people to those buyers. It is also the reason a couple local "flippers" call me first with every machine they have for sale.
I don't only make "better offers" beyond asking price just to get in the door - very often I give people more than we had agreed on when I see something in person that is better than I thought it would be. So I don't do it just to get in the door - I do it even after a deal has already been made just trying to treat sellers fairly.
Karma is an interesting thing. In life I have found "doing the right thing" always seems to pay off in the end.