An increasing number of enterprises are getting interested in exploiting knowledge, tacit or explicit, lying outside their organizational borders and augmenting the knowledge network of their organizations. A first generation of knowledge e-marketplaces has arisen to provide the platforms for knowledge exchange and trading in an inter-organizational level. This paper develops a framework to evaluate the business models, roles, processes, and revenue models of knowledge trading platforms and provides a survey of six existing knowledge marketplaces based on this framework. Finally, a set of conclusions is drawn on what issues should be addressed in a knowledge marketplace in order to eliminate the risks and gain the trust of its targeted customers.