In a mature market sellers are competing for a fixed number of customers. For example, a grocery store that wants to increase sales (above local population grow

In a mature market sellers are competing for a fixed number of customers. For example, a grocery store that wants to increase sales (above local population growth) must "steal" customers from its competitors. So my gain of a customer constitutes your loss of a customer, i.e. zero sum. Sellers achieve this (take customers from competitors) by offering higher value and/or decreased costs as I also mentioned.