Trust development among traders in an ad hoc m-commerce trading system is vital to mitigate uncertainty and risks involved in transactions. It helps traders decide whether to trade with potential trading partners as well as to gauge the degree of confidence that they should give these parties. One way to facilitate such trust is through use of a reputation system. However, the potential for ill-intentioned traders to subvert the reputation system makes the task challenging. This paper discusses key issues in designing a reputation system that can effectively facilitate trust development in such a loose and dynamic trading community. It proposes a fully distributed approach that employs a sanction-backed mechanism to encourage traders to be truthful in providing reputation reports. It advocates letting traders maintain their own reputation information as well as share knowledge about other traders' trading behavior in a peer-to-peer (P2P) manner without relying on network services that are always available. A security analysis on the proposed design shows that it can help traders detect or mitigate the identified misbehavior-related threats to a sufficient degree.