Abstract: Behavioral characteristics of the customers during group-buying auction are discussed to find out the influencing factors on customers' participation in group-buying, where seller's appropriate pricing policy can attract more customers. A pricing model is developed on the basis of an independent private evaluation model and some assumptions to maximize seller's profits, with a CPSO (chaos particle swarm optimization) solution given to it to seek optimal prices. Simulation results revealed that the markdown gradient in group-buying activities may affect the final gross sales, and the adjustment of markdown gradient will improve the values of customers' effectiveness. More customers will therefore participate in group-buying activities to increase sales so as to increase the profit of companies.