There was an article written up about ebay and how they paid 20 million$ per year for that first google slot for certain keywords.
Then some consultant came to Ebay and proved, without a doubt that the ads don't work. So, they created a test: they removed the 20 million $ worth of ads for that top slot and watched the traffic afterwards, it was pretty much unchanged! It's because All those people that clicked on those ads, were going to come to ebay anyways, despite the ads, not because of the ads.
Then some consultant came to Ebay and proved, without a doubt that the ads don't work. So, they created a test: they removed the 20 million $ worth of ads for that top slot and watched the traffic afterwards, it was pretty much unchanged! It's because All those people that clicked on those ads, were going to come to ebay anyways, despite the ads, not because of the ads.