Competitions that directly pit software agents against one another have proven to be an effective and entertaining way to advance the state of the art in a multitude of AI domains. Less frequently, human-agent competitions have been held to gauge the relative competence of humans vs. agents, or agents vs. agents as measured indirectly by their performance against humans. We are developing a platform that supports a new type of AI competition that involves both agent-agent and human-agent interactions situated in an immersive environment. In this competition, human buyers haggle (in English) with two life-size AI agents that attempt to sell them various goods. We describe several research challenges that arise in this context, present the platform architecture and accompanying technologies, and report on early experiments with simple agents that establish feasibility and suggest that human participants enjoy the experience.