The Internet of Things (IoT) is faced with a plethora of platforms which are built and operated as siloed solutions. However, the lack of interoperability and collaboration between platforms will negatively influence their adaptability to support future business cases targeting large-scale and cross-domain IoT deployments. While syntactic and semantic interoperability, as a prerequisite for platform cooperation, has been widely addressed by standardization and research actions, organizational interoperability enabling direct and business-driven interactions between IoT platform owners has so far not been in focus. In this paper we present the interoperability approach pursued by the project symbIoTe that implements a flexible interoperability framework, and put forward its solution for organizational interoperability offering decentralized and secure interworking between IoT platforms—IoT-platform federations. Three main collaboration mechanisms are introduced and elaborated: management of Service Level Agreements (SLA) between federated platforms, multi-level trust and reputation management, and a bartering mechanism for fair sharing of federated resources. The aforementioned mechanisms are put in place to foster decentralized platform collaboration which has higher potential to be adopted by platform owners compared to centralized approaches.