I have a buddy from the Erector set collecting community who has a pretty good approach. He goes to lots of toy shows/flea markets. When he sees something obviously way over-priced, he says something like "I don't mean to insult you, but based on other similar items, I think your price should be more like $xxx. I'd offer you $xxx." If they don't take it, he replies with, "Thanks for your time, wish you luck on selling your item, here's my contact info if you don't sell it".
Bruce