If your players are anything like mine, characters’ shopping can be a longest, most boring part of your D&D session (perhaps second to creating the characters themselves).
There is no Walmart (with exception to a wonderful dungeon I am thinking of designing) in D&D. Think about it, what sort of blacksmith could have created EVERY type of weapon and have one available in his shop? Who else is buying that stuff?
My method is to write out “shopping lists” on index cards before the game – labeling them with Standard, Specialized, Expert, and Elite tags based on the shop owner and location. Below are some examples of these categories and what their inventories may include.