Early success enabled Dell Computer to concentrate on its direct sales approach;selling computers over the telephone.According to Michael Dell,''Customers tell us what they want and we build computers to meet their needs and ship them out,usually within thirty-six hours.''By selling direct to customers,Dell avoids the price markup that retailers charge;in many cases,customers can purchase Dell's ''customized''computers for 15 to 25 percent less than the brands sold in stores.In addition to lowering prices,direct selling has enabled Dell to reduce inventory costs and to stay in touch with its customers.By constantly talking with customers over toll-free phone lines,Dell Computer finds out what customers like and dislike.As a result,the company has incorporated into its new product lines numerous small improvements that give it an edge on its competitors.