Online sellers must consider Amazon sales rank in making their buying decision. The rank is a measure of how often a particular book sells compared to every other book in Amazon's catalog of more than 3 million titles.

The sales rank can be as low as 1 or higher than 3 million. The lower the number, the greater the sales volume — a sales rank of 1 designates the top-selling book on Amazon. If you list a book for sale in the lowest 2,000 sales rank, the odds are it will sell within hours, assuming it's priced competitively.

You can look up a title's Amazon sales rank by looking in the "Product details" section of the title's Web page on Amazon. The number can change frequently, since Amazon recalculates the sales rank of books once every hour. There are also books with no sales rank on Amazon, which means that no copies have sold.

Since Amazon has an estimated 70-percent share of the Internet book market, its sales rankings are the best free information about book sales. Amazon made a series of changes in October 2004 to make its sales rank system more accurate and transparent. The rankings now take sales of new and used Marketplace items and e-books into account.

All this does not mean you should rule out books with high sales ranks, say more than 400,000. If you're the only seller with a copy, you'll get the sale. You might have to wait a year for a buyer to come along, but if you price high, it will be worth the wait.

Amazon does not publicly discuss its sales figures for individual titles, but a number of outsiders have attempted to calculate what a given sales rank means in terms of quantity sold. A book with an average sales rank of 1,000 sells about 90 copies a week, while a book with an average rank of 500,000 sells about one copy per week, according to statistics compiled by Morris Rosenthal, publisher of Foner Books. He figures that an Amazon sales rank of 10,000 translates into two sales per day, and a sales ran

k of 1 million translates into a single sale every other month.

In other research, economists at MIT figured that a book ranked number 10 on Amazon sells about 5,000 copies on Amazon each week, and a book with a rank of 100,000 sells 1.6 copies per week on average. Amazon sells more than 100 million books per year, and about half of those unit sales come from sales of titles ranking above 40,000, according to the MIT research. Titles ranked from 100,000 to 200,000 account for just 7.3 percent of sales at Amazon, and titles ranked from 200,000 to 300,000 produce just 4.6 percent of sales.

Slower-selling books tend to have higher prices, according to the MIT researchers. They found that the average price of books with a sales rank higher than 100,000 is about $8 higher than the faster-selling titles.

Title Z is a fee-based sales charting service that allows users to instantly retrieve historic and current sales rankings from Amazon and create printable reports with 7-day, 30-day, 90-day, and lifetime averages. The idea is to see how topics or titles perform over time compared to competing titles.

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