Following Thomas Aquinas, Francisco de Vitoria's analysis of justice in exchanges takes place by commenting on the corresponding questions of the Summa Theologica. The identification of the just price with that of common estimation occurs under a sufficient concurrence of sellers and buyers. A high level of concurrence limits the ability to take advantage of the need on the other side of the market. This fact guaranties a full consent of the parties involved in trading. Under conditions of market power or when some authority fixes a legal price, just price should also be taken as a normative ideal.