Electronic marketplaces are built to resemble real marketplaces structurally. Consequently, they are centralized systems, walled gardens with an intrinsic tendency to lock merchants and clients in. We argue that this structure is not necessary on the Web and that all online marketplaces could merge into one global medium for exchange. In this paper, we propose an architecture for such a medium based on semantic Web standards, encompassing the functionalities of publishing an intention to buy or sell, finding transaction partners, and conducting transactions. We focus on the basic protocol layer and explain how messaging and linked data are combined in a novel way to realize a highly dynamic communication system.