Agent-mediated e-commerce (AMEC) transaction services will be a paradigm shift from the existing client-server e-commerce model. In order to fulfill the leverage of AMEC intermediary services with secure and trusted service capabilities, we propose an agent-oriented public key infrastructure (PKI) operating with a variety of digital certificates. Under this agent-oriented PKI, several trusted AMEC transaction service models will be demonstrated using human and agent certificates showing, delegation, and verification protocols. We establish human/agent authentication, authorization, delegation, access control, and trusted relationships before these trusted AMEC intermediary services can be realized. This paper shows that a trusted AMEC system can be implemented in the FIPA compliant multi-agent system.