Need help making a shopkeeper character!

That is like ff7, in Dragon Warrior 4, you play through many chapters. in the first you play the main character etc. etc. In chapter 4, you play a guy who works as a merchant. People come in and sell you weapons and armor and sometimes you get a rare weapon. When a customer comes in, they want to buy something and offer a price. You choose whether to sell it to them or not. Sometimes they offer higher amounts than what its worth and sometimes they want the item really cheap. When you leave there, you get to keep any items that weren't there when the chapter started(like this very strong weapon a knight sells you) as long as you don't sell them. I want the player to be able to build an entire city and name it and everything. That will be the best mini-game/side-quest in the world! Thanks Bluecube!