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I'm looking to create a website selling a specific niche range of products. For example 'hair extensions.' Using opensite explorer it's fairly easy to guage the competition for similar websites focused on this niche only e.g www.hairextensionsworld.com  - I just look at their authority, number of incoming links, anchortext links etc. However, I'm finding it more difficult to guage the competition if a large merchant, for example Amazon, has a page ranking for a specific keyphrase I'm after.  Often the large merchant doesn't have a lot of external links to the specific page, but nonetheless it has power from being on the domain of the large merchant. Are there any good tools / metrics for guaging the competition of such pages? Page authority I guess is one? Thanks