Simply select Zin Bu from the followers screen to enter his store.
In-Game Description
Zin Bu the Magic Abacus is a celestial bureaucrat who seeks to shift over to sales. Check back often; his magical store occasionally restocks itself to help you on your journey.
Notable Game Influence
Zin Bu is a very useful tool in Jade Empire, as he allows you to buy and sell equipment at any time. While you won't get as much money for your gems when selling to him, he does stock a nice assortment of gems, techniques, and styles during each of the game's chapters after he's joined as a follower. Here's a full list of what you can buy from him, and when:
