[matthewmacleod]

But I don't think they are different markets. If when you search for a web page google shows you the best result, why shouldn't it search for the best/cheapest online retailer? It's searching for the most relevant 'thing' on both cases.

I don't think that would be a problem, but it's not what happened. Google has a distinct 'shopping' product, and retailers have to pay to place their product in there. Almost by definition, this means that ranking is broken, and Google is essentially running its own comparison site where retailers pay for listing.