With panning shots, you are only going to get perfectly sharp pans, when the object is moving perpendicular to the camera.
something like this...
If the relative velocity of the car is moving towards or away from you, it will always appear soft because teh car is actually moving away, and only one point will appear sharp.... like the number 36 in that 4th shot.
Only problem is, side shots of cars are pretty boring, and don't show alot of action.
So its all a trade off.
I havn't sold any images yet, i wouldn't know where to start with how to price and advertise.