This volume contains 18 thoroughly refereed and revised papers detailing recent advances in research on designing trading agents and mechanisms for agent-mediated e-commerce. They were originally presented at the 11th International Workshop on Agent-Mediated Electronic Commerce (AMEC 2009) collocated with AAMAS 2009 in Budapest, Hungary, or the 2009 Workshop on Trading Agent Design and Analysis (TADA 2009) collocated with IJCAI 2009 in Pasadena, CA, USA.The papers focus on topics such as individual agent behavior and agent interaction, collective behavior, mechanism design, and computational aspects, all in the context of e-commerce applications like trading, auctions, or negotiations. They combine approaches from different fields of mathematics, computer science, and economics such as artificial intelligence, distributed systems, operations research, and game theory.