Back in the olden days, merchants carried their products in a horse-drawn wagon from one location to another. Sometimes they just visited the local stores in a town, and other times they went door to door. They kept track of their own merchant account as they dealt with each customer, either demanding a cash payment, an item in trade, or perhaps letting them “put it on a tab” if times were hard.
It’s really not so different today. Merchants still carry their products from one location to another, but they do it faster than any horse power could have ever provided. They keep track of their own merchant account, but with the assistance of a company who lists each transaction and gives them a total at the end of the month (or daily, if needed.) They still want the cash, but in lieu of actual greenbacks, they let the customer “put it on a credit card” which they can pay to a third party later on.