See it’s called “customer” because you’d be “giving custom” by customarily getting your iron from this one forge or having a standing order with this cheesemaker for X pounds each winter, because you might buy one-off finished consumer goods from artisans or marketplace traders, but established merchants dealt in bulk staples and intermediary goods

The RPG “item shop” is a backwards projection of a general store concept that dates to the 19th century

(Potion shop actually has earlier precedent)