This paper presents an auction system based on mobile agents. By using a proxy server, a user can generate a mobile auction agent by giving the bidding information, including the maximum bidding price. The agent then moves to the required server to participate in the auction according to the user's requirements. A major contribution of the paper is that it presents a mathematical model to analyze the commonly used "proxy bidding method." Based on the distribution of the maximum bidding price, the model can be used to find the probability of stopping at a certain price and, hence, the average winning price. Simulation and analytical results are presented to demonstrate the behavior of the system.