In order to trade with townspeople you need trading items that are different for each person. For example, to trade with the Flower Girl you need to craft a Magic Pot with the Fortune Teller, find the Flower Girl on the map, and tap the "Trade" button. If you do not have a trading item you can "Find" it or "Buy" it for rubies. If you tap the "Find" button a blue arrow will show you where to find the item. All townspeople can be found on the Darkwood map.