In the business world a tire kicker is a party who wants to come look at your deal to learn what might be learned, but has no serious intention of taking part.
My impression with regard to the use as a description of a potential automobile purchaser is that it describes someone who has no idea what to look for wanting to appear as though they are inspecting the merchandise. You can actually detect looseness (worn tie rod ends or some such) in the front end with a push on the wheels, probably better done by grabbing the tire with your hands and wiggling it.