We have designed, implemented, deployed and evaluated a large-scale agent-oriented information system that recommends relevant documents to users. Our recommender system is now being used across several European institutions. Its two key features are a modular design capable of accomodating multiple recommendation methods, and the use of a marketplace to select and rank the best recommendations for the user. As part of our evaluation, we have extensively simulated this marketplace in order to understand its dynamics and validate its suitability for a recommender system.