The Internet of Things (IoT) paradigm has been rapidly adopted in a plethora of application domains, promoting the development of a variety of smart services relying on data from different sources. As a consequence, Sensing-as-a-Service is also gaining popularity, enabling IoT data providers and consumers to share, exchange and trade IoT data. In such a data-driven ecosystem, data marketplaces play a pivotal role, which besides the need for interoperable solutions, necessitates also sophisticated market mechanisms that ensure trusted and secure data exchange. In this paper, we present the IoTFeds solution which offers a complete open source interoperability framework combined with blockchain technologies, and thus provides a decentralized federation management and marketplace platform. The IoTFeds platform enables trustworthy and secure transactions allowing the creation and monetization of composite IoT data services offered by multiple federated data providers.