All puzzles are solved by trading and combining items.

Controls

• Click / Tap
  • to focus on item
  • to give or combine focused item
• Held Click / Long Press 
  • Ask an NPC what they want
• Alternatively, drag-then-drop to focus-then-combine in one action

P.S.

A "Jam Reviewed" version is tagged and available on GitHub. 

https://github.com/lunarcloud/exchanges-puzzle-game