In the state of Kansas, dealer's cannot broker cars, only buy and sell them.
So we can "buy" them and not pay until the car is sold
... Kind of a silly rule.
Barrett-Jackson charges a 6% to the buyer and 6% to the seller for an auction sale. It was the same rate when I sold my '84 500SL at the blacktop nationals auction which was led by Spanky himself.
http://www.youtube.com/watch?v=b7CetpZjh_0
I would imagine it would depend on the car being sold. If its a 6 figure car I can't imagine someone asking more than a single digit percentage. F40 has quite a selection all over the place price wise, so I imagine there's a sliding scale.