You meet the Trader upon entering the Endless Islands. You can exchange goods from him for a special currency: Opals.

Where to find opals?

Opals can be obtained from special chests. Use hints in the resource menu to locate them.

What goods are there?

The number of goods available for purchasing from the Trader is limited. However, the goods are restocked after starting a new island.
The Opals collected on previous islands can be spent at any time in the future.