Credit - https://www.reddit.com/r/DnD/comments/3mvmso/5e_dd_shopping_catalog_i_made_for_ingame_use/
The idea behind this shopping catalog represents what a reasonable person living in this world would expect to find in a shop of that type in that location
It gives a reasonable idea of the prices a shop has compared to an average going rate, and how much a shop-owner could reasonably buy without making a persuasion check, as well as how much he could possibly buy a day from you without trade. You could even try to become better friends with the shopowner help the chances of lowering the price, or perhaps doing a quest for him will accomplish that.