Tribler developer here. We have been working hard to mature and deploy this feature, and we are getting close to a fully operational token economy and decentralized marketplace. We have designed and implemented own scalable blockchain implementation, which we use to keep track of bandwidth transfers over time. The next step is to actively refuse connections to peers who have a low balance.
Other than that, we are also actively working on many other scientific problems. One of these problems, is how to bootstrap trust in distributed systems. Numerous master students and PhD students have been working on this topic. For a complete list of our ongoing efforts, see https://github.com/Tribler/tribler/wiki#current-items-under-....