flour dealer; flour shop
Refers to a person or shop that sells flour. Can also be used for a business that deals in flour products.
miller
Specifically a person who operates a mill to grind grain into flour. Less common than the dealer sense in modern Japanese.
Refers to the facility or building where grain is ground into flour. Often used in historical or rural contexts.
My grandfather ran a small flour shop in town.
The miller was grinding wheat using a waterwheel.
An old flour mill still remains in that village.
From 粉 “flour, powder” and 屋 “shop, dealer.” The reading こなや contains no sequential voicing.