Once per day, or three times for VIP members, you can customize your trade with Chanya by selecting an item that you would like to get rid of in exchange for an item you would like to acquire. Keep in mind that once you choose your items, you will be locked in for that trade. If you press the Cancel button this will still count as your custom trade for the day, but don't worry, you won't lose your resources. If you're not completely sure, you can press the X in the top right corner to close the trade menu, and then reopen it later to confirm or cancel the trade.