In agent-mediated marketplaces, autonomous agents deploy automated bidding mechanisms in order to increase revenue for humans. The ability of agents to estimate the next prices to be revealed in an auction, by applying forecasting, is a key element for efficient and successful bidding. In open outcry auctions, such as English and Dutch, information about bidders behavior is revealed at each round. This paper proposes a bid calculation function based on forecasting of the next price in English and Dutch auctions. The forecasting is based on two linear adaptive filters for stochastic estimation, whose parameters are calculated using a genetic algorithm. In order to test the efficiency of the two bidding methods and to benchmark the performance of the two filters, we conduct a set of experiments and present the results.