Quality certification programs help consumers identify high-quality products or sellers in markets with information asymmetries. Using data from eBay UK's online marketplace, we study how certification's impact on demand varies with market- and seller-level attributes, exploiting variation in sellers' certification status within groups of near-identical listings. The positive effects of eBay's “top rated seller” certification are stronger for categories with few other certified sellers, in more competitive markets, and for sellers with shorter records of past performance. These findings indicate certification provides more value when certification is rare, the product space is crowded, and for sellers lacking established reputations. (JEL D12, D82, L15, L86)