To support the purchasing process in the B2B EC platform, we analyzed various architectures of buyer-carts. The desired features in buyer-carts are collection, recording, trashing, tracking, identification, ordering, payment, purchasing decision support, and transmission of records to e-procurement systems. A buyer-cart can be categorized as s-cart, i-cart, and b-cart depending upon its residing sites: seller, intermediary, and buyer. To design the architectures of B2B e-marketplaces considering the role of buyer-carts, we analyzed the feasible combinations of marketplace operators, pricing mechanisms, and types of buyer-carts. Seventeen elementary types of B2B EC architectures turned out meaningful in this regard, thus their pros and cons are evaluated. Based on this framework, we designed a composite architecture MyCart, which allows the buyer to use b-cart along with s-cart and i-cart. By using the b-cart, a buyer can visit multiple sites collecting information in his or her own cart. This will allow the tight integration of b-cart with the e-procurement system. We demonstrate that the b-cart approach can be an effective framework of integrating the e-marketplaces with e-procurement systems and ERP systems. The b-cart can be very effectively applied to the desktop purchasing environment which uses external e-marketplaces.