Agent based online auctions have not yet become popular because they are not trustable. One of the major concerns in agent based online auctions is the shilling behavior problem, which makes winners have to pay more than what they should pay for auctioned items. In this paper, we propose a real-time trust management module for agent based online auction systems using rolebased access control mechanisms. As one of the key components of the trust management module, a security agent can actively monitor online auctions in order to detect abnormal bidding behaviors in real-time. To illustrate the feasibility of our approach, we implemented a prototype real-time trust management module for agentbased online auction systems, and demonstrated how shill agents could be efficiently detected.