New York--American Express Co. and Mountain View, Calif.-based Ventro Corp. will create MarketMile L.L.C., a company that will launch a b-to-b procurement exchange. MarketMile will initially target midsize businesses with everything from office supplies to computers to temporary labor. Later, American Express will pitch the exchange to small and large businesses. The idea is that American Express will provide marketing and b-to-b payment muscle, while Ventro will supply exchange-running know-how. Mountain View, Calif.-based MarketMile's exchange will also be offered to Ventro's operating companies, such as Chemdex and Promedix. Ariba Inc. will help build the exchange.