I'm completely jumping to the end of this thread without reading what's already been posted, so please forgive me if this has already been said.
If your intent by posting is truly to "educate a seller" do it via PM. The only point for making it public would be to prevent the seller from selling the game at his stated price. Additionally, I don't think it has to be enforced both ways. That's like teaching you're kids that it's okay to insult someone because it's okay to complement them. Obviously that's just not true.