See it's called "customer" because you'd be "giving custom" by customarily getting your iron from this one forge or having a...
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 or 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)
I do appreciate how in Jeff “Spiderweb Software” Vogel’s Exile/Avernum tactical CRPGs the “general merchant” role was often filled by “quartermaster of an outpost, trading from its stores”, that’s pretty historically plausible
