During the months since "B2B" (business to business) became the password to success on the World Wide Web, conventional wisdom has shifted at least twice. In the beginning, everyone seemed to believe that the way to flourish was to become an independent on-line market maker. Companies such as PaperExchange and e-Steel quickly set up shop using a readily understood business model: capture a significant share of a particular B2B market, charge a small fee for matching up buyers and sellers, and watch the revenue pour in.

By some estimates, more than 1,000 such e-marketplaces—for products that ranged from commodities such as lumber to specialized components such as airplane parts—managed to receive funding. Unfortunately, most of these companies failed to realize that the lifeblood of a marketplace is liquidity and that, in B2B, a few large enterprises can generate most of the transaction volume so critical for that purpose. These behemoths typically don’t need the help of an independent marketplace, however, and they can bargain fiercely with anyone who hopes to trade with them. Independent, fee-based marketplaces have therefore mostly languished in the absence of a business model that could vindicate their early optimism.

In the second wave of B2B,...