As a massive amount of Internet of things (IoT) devices are being deployed for data collection, it is desirable to have a marketplace where both IoT device owners and data consumers can trade collected data freely and effectively. While it is possible to build a centralized data trading platform similar to existing commodity markets, it faces several limitations such as lack of neutrality and dependence on external payment system. The development of blockchain technology sheds light on addressing these concerns, and several works have been done on developing blockchain-based IoT data trading systems. Introducing the blockchain into the IoT data market construction also brings new challenges including management and trustworthiness of off-chain information, and supporting rich and fair trading options in a decentralized environment. We propose DTIDM, a decentralized trusted IoT data marketplace, to mitigate these issues and stimulate the trading of IoT data to maximize its potential value. DTIDM utilizes a hardware-software hybrid approach to address the trust issue in data trading and supports various trading mechanisms like selling, direct exchange, and other user-defined trading types. The design of DTIDM is decoupled from the underlying blockchain, so both the framework and key components of DTIDM can be leveraged in an environment selected by the user. We also develop a prototype of the trusted IoT data provider based on the DTIDM design.