I once worked at a company that did something very similar to what you’re suggesting. It doesn’t work as well as it sounds. The problem is that the feedback m

I once worked at a company that did something very similar to what you’re suggesting. It doesn’t work as well as it sounds. The problem is that the feedback mechanism is far too easily compromised (both by intentional means and unintentional ones). In order for such a scheme to function properly, the ad network has to be able to tell how many sales were made as a direct result of the click. The feedback loop needs to be nearly perfect. As things stand, it is terribly easy for ad purchasers to game, and not everybody is honest. I agree that the concept is ideal, but I have yet to see an implementation that actually works without a great deal of additional effort (and a lot of human attention, and many many arguments…).