As data marketplaces are becoming common place, it is also becoming clear that data streams generated from Internet of Things (IoT) devices hold value for potential third party consumers. We envision a marketplace for IoT data streams that can unlock such potential value in a scalable way, by enabling any pairs of data providers and consumers to engage in data exchange transactions without any prior assumption of mutual trust. We present a marketplace model and architecture to support trading of streaming data, from the advertising of data assets to the stipulation of legally binding trading agreements, to their fulfilment and payment settlement. Crucially, we show that by using blockchain technology and Smart Contracts in particular, we can offer participants a trade-off between the cost of transactional data exchange, and the risk of data loss when trading with untrusted third parties. We experimentally assess such trade-offs on a testbed using Ethereum Smart Contracts.