I have officially established that Pay Per Click advertising on Google and Yahoo, and other Product Search engines like Shopzilla, Bizrate, Pricegrabber, etc are a complete waste of money. For the past 2 weeks, I put about $150-$200 spread out across 5 or 6 different websites like that and discovered them to be nothing but garbage.
I would guess that only about 75% of the clicks they say I had, actually made it to my website... the rest just disappeared without any trace, except for the promise by these companies that the people were real.
Of all the hundreds and hundreds of clicks that I supposedly had, I made "0" sales. I make a ton more money from CraigsList. Despite eBay's outrageous fees, at least they brought in several sales.