With the increasing needs of purchasing from e-marketplaces, desktop purchasing, intelligent agent for B2B EC, we show a b-cart based agent system framework which is efficient in purchasing from multiple e-marketplaces, In this paper, after describing the concept of b-cart, a buyer-cart located on buyer site, we analyze the roles of buyer agent, seller agent, and intelligent agent in B2B EC and Agent Communication Language. With the consideration of such roles, we propose the thirteen features of b-cart based agent system in B2B EC; Identification, User Dialog, Collection, Trashing, Individual Purchase Decision Support, Organizational Purchase Decision Support, Negotiation, Ordering, Payment, Tracking, Recording, Record Transmission, Knowledge Maintenance. Based on these features, we design the buying process, message interfaces, and analyze the pros and cons of this system. Even though the b-cart based agent system has some overheads such as in development, operation, and deployment, the buyer using a b-cart can collect items from different e-marketplaces, and can order purchases simultaneously over the collected items. The b-cart based system also supports integrated services in personalized comparison on purchasing items, order tracking, financial/payment management and users account management, negotiation management, personal decision support along the organizational decision making process. This allows a tight integration of the b-cart with e-procurement systems. Since the number of e-marketplaces and individual buyers in an organization increases, we expect the effectiveness of the b-cart based agent system will increase in B2B EC.