In this paper, we propose a novel incentive mechanism for eliciting fair ratings of selling agents from buying agents. In our mechanism, buyers model other buyers and select the most trustworthy ones as their neighbors from whom they can ask advice about sellers. In addition, however, sellers model the reputation of buyers. Reputable buyers always provide fair ratings of sellers, and are likely to be neighbors of many other buyers. In marketplaces operating with our mechanism, sellers will increase quality and decrease prices of products to satisfy reputable buyers. In consequence, our mechanism creates incentives for buyers to provide fair ratings of sellers.