Abstract
This paper examines how to design token-based favor-trading platforms when users constantly join and leave and differ in how often they participate and how much they value services. We propose a simple mechanism with three rules: users enter with zero tokens, they can hold and redeem tokens only up to a finite cap, and any remaining tokens are recycled when they exit. These rules keep token circulation stable and make it optimal for users to always be willing to provide services, as long as the benefit of receiving help is sufficiently larger than the cost of helping others. We show that a finite cap is necessary for such an equilibrium to exist, and that token recycling prevents instability as the platform scales. The mechanism remains effective under substantial user heterogeneity, offering practical guidance for time banks, decentralized capacity-sharing platforms, and other tokenized service-exchange systems.