  Motivated by applications such as voluntary carbon markets and educational
testing, we consider a market for goods with varying but hidden levels of
quality in the presence of a third-party certifier. The certifier can provide
informative signals about the quality of products, and can charge for this
service. Sellers choose both the quality of the product they produce and a
certification. Prices are then determined in a competitive market. Under a
single-crossing condition, we show that the levels of certification chosen by
producers are uniquely determined at equilibrium. We then show how to reduce a
revenue-maximizing certifier's problem to a monopolistic pricing problem with
non-linear valuations, and design an FPTAS for computing the optimal slate of
certificates and their prices. In general, both the welfare-optimal and
revenue-optimal slate of certificates can be arbitrarily large.
