A planning agent supporting customers plays a distinguished role in internet marketplaces. Although several internet marketplaces have been built with the maturity of tools based on internet and distributed technologies, there has been no actual study up to now with respect to the implementation of the planning agent. This paper describes the design and implementation of an XML-based planning agent for internet marketplaces. Since implementing internet marketplaces encounter problems similar to those in other fields such as multidatabase or workflow management systems, we first compare those features. Next we identify functions and roles of the planning agent. The planning agent is implemented using COM+, ASP, and XML, and demonstrated using real data used in an existing system.