By trading with the people of Darkwood you can get energy, fixers required for combining collections, and weapons for banishing or placating creatures.

You can trade with the Flower Girl, Woodsman, Sweep, Dancer, Violinist, Actor, Seamstress, and Lady.

You can find them moving around the Darkwood map. Townspeople with whom you can trade appear as the plot develops.
In order to trade with each townsperson you need different trading items. 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.
You can also "Buy" a trading item for rubies or "Find" it. Tap the "Find" button and a blue arrow will show you where to find the item.