right goat, it depends on the car you are selling. For a G I think BAT works great, but for many special interest cars, these large auctions work very well too. Auction houses compete on pass-thru percentage rates, but these rates are always "across the board" rather than specific to certain vehicle types or marques which can be rather miss-leading.
I know a few collectors and dealers that have done well and poorly at the big auctions. Our own club guy Norm has sent cars down sometimes several at a time and have them not sell (huge loss) but other times big wins. With the right egos in the room the prices can go way overboard. So if you have a car that ego-maniacs love to show off in, BJ etc is the place to put it. AKA muscle cars as example, customs, resto-rods, etc.