I'm an Amazon (AMZN) shopper (my Kindle 2 is supposed to arrive today!), but I was still surprised to read that the company could be responsible for one out of every three online purchases. Barron's quotes RBC Capital analyst Stephen Ju as estimating that Amazon's direct sales and the value of third-party sales made through the site added up to 34 percent of all internet sales. The U.S. Census estimated that e-commerce retail sales last year totaled $103 billion dollars, meaning Amazon touched $44.5 billion worth.

Last year, Amazon reported sales of $19 billion. How much of of that comes from third-party sales, from which Amazon takes a fixed fee, revenue share fee, or per-unit activity fee, is a point of great speculation. Ju's estimate of a 34 percent share represents an increase of 7 percent over the previous year. Amazon's net sales were up 28 percent during the same interval, although its inventory turnover was down slightly, from 13 to 12.

Amazon has excelled in employing the three pillars of e-commerce: logistics, databases and internet purchasing; so the fact that sellers are eager to join its program should come as no surprise. However, the company is in an awkward position, in that companies whose products find a strong market may well choose to abandon the Amazon relationship. It also has to accept the hazard of depending on other companies to keep adequate inventory and meet the delivery and customer service promises.

Like Google (GOOG), however, Amazon at present has no real competition. Behind it are Staples (SPLS) and Office Depot (ODP), with only about a third of the sales volume apiece. In an economy where price rules, Amazon's ability to undercut the brick and mortar stores should bode well for 2009.