It would be pretty hard to figure out. One thought occurs to me is to poll an area to get some pinballs/household insight, and then determine the correlation to the volume of sales activity on CL and ebay.

You would have to poll several areas of varying demographics (large urban centers on the coasts, lower density population areas (flyover states), and some in between. But ultimately what it would mean is, when there are x number of pinheads in an area, there tend to be y number CL/Ebay ads. Therefore, if I see y number of CL/ebay ads in a random unpolled area, there are likely x number of pinheads.

Probably a more accurate way is to go by Stern's sales numbers instead of CL/Ebay, but of course, that information is not public.

There is probably also a pretty tight correlation between purchasers of video pinball on xbla and psn and the total pinheads in an area, but again, not public info.