This volume contains 9 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 12th International Workshop on Agent-Mediated Electronic Commerce (AMEC 2010), collocated with AAMAS 2010 in Toronto, Canada, or the 2010 Workshop on Trading Agent Design and Analysis (TADA 2010), collocated with EC 2010 in Cambridge, MA, USA.The papers examine emerging topics such as ad auctions and supply chains, or the interactions between competing markets, and present novel algorithms and rigorous theoretical results. Several of them evaluate their results using real data from large e-commerce sites or from experiments with human traders.