Data is currently perceived as one of the most valuable resources by the industry. In this context, data marketplaces have emerged for facilitating data trading in a coordinated manner. To facilitate open, fair, and transparent trades, it is necessary to ensure trust, both among the different participants and stakeholders, and trust in the sense of confidence to the ecosystem. Thanks to the emergent blockchain technologies, there are new decentralized data marketplaces that provide this confidence in a decentralized manner with new disruptive services such as automated conflict resolution, non repudiation and auditable accounting. In this paper, we present a decentralized data marketplace called i3-market that implements these services. We also describe a service called free data sampling that allows a consumer and a provider to securely agree on a portion of a dataset that can be checked by the consumer before committing the purchase. Finally, the paper discusses how to integrate the free sampling service in the i3-market ecosystem.