Using Visa debit and credit card transactions in the U.S. from 2017 to 2021, we document the importance of customers in accounting for sales variation across merchants, across stores within retail chains, and over time for individual merchants and stores. The number of unique customers, as opposed to transactions per customer or dollar sales per transaction, consistently accounts for about 80% of sales variation. The top growing and shrinking merchants account for the majority of total sales reallocation over time, through their acquisition and loss of customers. To illustrate potential implications, we write down an endogenous growth model with a customer margin. In this model, the customer margin drastically increases the size of the largest retailers and their contribution to overall growth, boosting aggregate innovation and growth in the process.