Your putting too much faith in the buyers of ads reacting to the changes in prices and knowing exactly how much the traffic they are buying is actually worth. All mortgage broker's/free i-pod scam/dating site/etc know the exact ROI they get back from each and every little bit of traffic? No way. In some industries, theres even a bunch of buyers, some less sophisticated than others, but are yet still able to buy the traffic at a certain rate because they are making a profit at that rate. That isn't optimization. In industries with fewer buyers (high traffic dating sites), one site might have to purchase traffic from a multitude of different sources, or indirectly purchasing it from their own affiliates who might purchase it themselves, to keep up their market share against other big competitors.

This isn't about spotting temporary market inefficiencies, its about spotting long term ones that lurk beneath the surface.