The Whole Land Catalog

Writing Excuses Episode 14.7 asks audiences to do a homework wherein you identify "one buy," aka the one bit of worldbuilding weirdness that your audience will accept on faith without getting thrown off or out. In this world, Jack is a shopkeeper who travels around the realms of the land to identify interesting things to add to his shop's sales catalog--and, he hopes, to establish enough customers to run a mail-order business. Also the technology. The one buy is that elements (think the periodic table) exist in this world that interact with the classical elements: Earth, Air, Fire, Water, Metal, and Wood.