The advancement of Internet-based commerce has created a turbulent market environment by allowing easier introduction of new products, services, and suppliers. For this dynamic environment, information systems need to be designed in a flexible way to meet the changing requirements. This paper proposes a flexible transaction framework for multi-agent systems so that agent systems can adapt to various types of changes in the dynamic electronic market environment. The flexible transaction framework consists of a flexible conversation model (FCM) for agents and an ontology meta-model called TOO (Task-Oriented Ontology) for the specific requirements of FCM. FCM permits agents to use newly acquired conversation policies that are induced by the frequent changes in transaction methods. Since FCM requires agents to be aware of a broader set of ontology entities, TOO is used to reduce the complexity of the ontology design and implementation for FCM. The flexible transaction framework is illustrated with an online traveling system developed for Korean domestic transportation services. The prospective synergy between the present work and Web services is discussed in the discussion section.