I am a car salesman too (mazda/infiniti). It really is amazing how people come in and think they deserve to get things at "invoice", yet the shirt they have on probably has a 500 percent markup, and they think they got a hell of a deal.
I don't even mind coming off sticker, but a fair deal is when both parties are happy. Plus, if I spend 5 days trying to get you to buy a 100 dollar car, chances are, your follow up customer service from me wouldn't be too good.. well for me it would becuase I know how to do my job, but for most people it wouldn't.
Also something else that gets me, is people are so focused on the invoice, they don't even realize, well alot of them at least, that the car lot's biggest cash cow is the back end of the deal. Interest Rate, F&I. You are so amped about getting your car at "invoice", you forgot to notice the extra 3 percent interest you are paying for the car, which on a 20 thousand dollar car is 600 dollars