Amazon also benefits greatly from its advanced inventory management methods and ability to negotiate beneficial payment terms with vendors. The company sells such a large volume of merchandise, and can predict customer demand so accurately, that it generally sells products within 65 days, before it has to pay suppliers for them.

That arrangement, which analysts call “negative working capital,” is unusual outside of grocery stores and allows Amazon to avoid the huge capital charges associated with buying and storing such a broad line of inventory. It also boosts the company’s cash flow, which it has used to pay down its debt to $109 million at the end of June from a hefty $2 billion in 2000, and to add more product lines to its Web site.

Amazon’s profit and margins have always been slender; it earned only $645 million in 2008, up 36 percent from the year before, compared to Wal-Mart’s $13.4 billion, up 5 percent. But Wall Street is more enamored by the promise of the online retailer, valuing Amazon at around 60 times earnings and Wal-Mart at 15 times earnings.

“They don’t have to incur huge inventory carrying costs and can add product categories almost ad infinitum,” said Jeffrey Lindsay, an analyst at Sanford C. Bernstein. “Amazon has an almost magical business model in terms of inventory management.”

AMAZON’S incursion into general retail has rivals scurrying to regroup and stop its advance.

In August, Target, which allowed Amazon to run its Web site for the last decade, announced it would end the affiliation when its contract was up in 2011, following other one-time Amazon partners like Borders and Toys “R” Us. This month, Wal-Mart said it would allow other retailers to sell their products on Walmart.com, mimicking Amazon’s third-party marketplace and trying to match its vast selection. Analysts believe Sears, which owns Kmart, is preparing to allow outside sellers on its sites as well.

But the Amazon effect may be most deeply felt by small independent stores, which cannot hope to compete with Amazon’s selection and prices and recall in fear how the company hastened the fate of both independent booksellers and prominent electronics chains like Circuit City.

Like many small business owners, Ken Lombardi, the C.E.O. of his family’s 60-year-old Lombardi Sports in San Francisco, views Amazon as a source of some of his business troubles. Though his store has been hit hard by the recession and the expansion of a sporting goods chain, R.E.I., in the Bay Area, Mr. Lombardi says that it is Amazon that has helped depress profit margins and snagged sales for basics like silicone swim caps, undershirts and running shoes  which, he adds, Amazon can offer without California’s 8.25 percent sales tax.