The problem: the owner is the whole back office
In a mom-and-pop shop or small trading business, one or two people do everything: serve customers, negotiate with suppliers, write quotations, raise bills, chase payments, count stock and answer enquiries after closing. Anything that is not urgent gets postponed, and the postponed items are usually the profitable ones. Quotations go out two days late, by which time the customer has bought elsewhere. Overdue payments are chased when cash gets tight rather than when they fall due.
Stock behaves the same way. The three items that sell every day are the ones that quietly run out on a Saturday, while slow-moving goods absorb capital on the shelf. The knowledge to prevent this is in the owner's head, not in any system, so it cannot be acted on when the shop is busy — which is precisely when it matters.