[kaczordon]

This doesn't clear anything up though. Who gets to define the boundary/scope of the market being measured? It's pretty easy to manipulate this to make it seem like a company has a "dominant" share even if that's not the case from the consumers point of view.

Take the anti-trust case against Microsoft, for example, the market was defined as that for computer operating systems for stand-alone personal computers using microchips of the kind manufactured by Intel. This left out not only operating systems running on Apple computers but also other operating systems such as those produced by Sun Microsystems for multiple computers or the Linux system for stand-alone computers. In its narrowly defined market, Microsoft clearly had a “dominant” share.

Also lots of people use Amazon to search for products so it's not clear to me how there is a market failure here.