HarveyTwoFace. I think you are overlooking a major idea that another posted about.
The auction house is telling you what they THINK it will sell for. The dealer you mentioned offered you actual currency. What if I told you I could market it to the optimal buyer, the perfect buyer, the buyer searching not only this year and mint-mark you are offering, but that they want THAT specific coin, and will pay $5k for it!! and I only want 20%! You would net $4k!! Obviously you would consign the coin to me right?
Personally I think the auction house was the more deviant in their pitch to you. But overall I think you illuminated your own ignorance by insulting a dealers integrity so blatantly who knows more about numismatics, as well as how the real market works.