Abstract: Interoperability between autonomous decentralized systems needs support of an information service that can provide dynamic knowledge about available service providers within the network. We describe trading service. Trading is an essential infrastructure function indicated by the ODP (open distributed processing) reference model, a meta model of heterogeneous distributed processing. The purpose of trading is to support location transparency of services: it hides the location of service providers, even if they are located in separate, autonomous domains. Cooperation between trading service providers (trader) supports federation transparency, i.e. hides differences of technology used in separate domains. In cooperation, the traders have to mediate service type differences, negotiate policy differences and tolerate technology differences between domains. Together with other fundamental functions trading offers a platform for open distributed computing between autonomous subsystems.