Um, a couple of times the dealer has offered better financing than our credit union. So we gave our credit union their check back. If we had just handed our check to the dealer and driven off we would have paid more.
In other words it it isn't the end of the world to hear what the sales staff can do. That isn't to say I haven't walked out on old school sales people or blacklisted an old school dealership. I don't like being treated like an idiot.
I am simply saying you might be screwing yourself by not listening to the sales people. Even when I am paying cash for our annual work truck purchase I will hear a good sales person out. It has paid off a couple of times with a better vehicle or deal.
A man who can laugh, if only at himself, is never really miserable.