[digging]

> and you cannot easily measure conversion

Easily, no. But you can, at least in theory. With enough data, you can essentially tell if a customer has seen your ads, plus when and where (and not just your online ads - billboards and dealerships could be identified by using the customer's car to track their location). If they buy your car, and you see that they viewed your ads, maybe they even clicked one or otherwise browsed your site at some point, you can get some data from that. With enough aggregate data, you can begin to see correlations between certain ad viewing behavior and certain purchases.

Do they actually do this? Probably not, due to incompetence. But they could and eventually will if they aren't already.

[wnevets]

I believe Google is/was trying to do that using your credit card payment history.