Drawing on a rich legal dossier on a violation of obscenity laws (the Trafford affair), this article studies the formation of a sex market in early twentieth-century Paris. Books and erotic photographs, contraceptives, sex toys and remedies for increasing sexual potency: the diversity of these products relating to sexual practices is clearly revealed by the case. The dossier demonstrates how this commercial enterprise drew on the benefits of industrialization, mass media and globalization for building its client-base and developing a transnational dimension. It also allows us to understand the strategies deployed to keep the sale of these objects clandestine, in a context where they were increasingly overseen by the authorities, revealing innovative forms of commercial exchange, based on an epistolary transvestism.