The idea with the merchants is you can get some goods, even if there are zero people to trade with. You just have to wait the 6 hours for the merchants to be available again at no charge to you, or use time boosters or diamonds to make them available right away. The 2nd main hall upgrade in chapter 19 raises the amount per trade to 5000 for same tier, and one of the AWs at the end of chapter 19 increases the amount also.