i'm researching some niches on amazon. is there a standard number i want to look for in the Amazon Best Sellers Rank? What number should it be below to make sense as a possible niche market? for example, the top book in one of the niches i'm thinking of is 2,000,000. that seems pretty high. any thoughts? thanks! p.s. one of the books in the list is 97,000 but it's only 99 cents.