I've wanted to build a decentralized market app/platform for a long while now. Reputation management/calculation and discovery are the biggest hurdles.

My initial desire was to build it for local food producers, like farmers, butchers, canners, etc. Neighbors would rate each other's processes/quality and certify/endorse health standards. As a result, a small community would accomplish for itself what centralized system takes forever to do. It also corrects the incentive structure in that a business becomes responsible to their neighbors and not the state.