A growing number of retailers are using software that changes online prices based on demand, competition, inventory and other factors. The main goal is to undercut rivals when necessary, and raise prices when demand is high and there's no competitive pressure. After years of losing customers to Amazon because of its ability to offer deep discounts, Wal-Mart and others have started following the online retailer's lead. Eric Best, CEO of Mercent Corp., a software company that changes prices on two million products every hour, said the majority of his clients that include Office Depot, Guess and HSN Inc.