From Guild Wars 2 Wiki
Trader's Keys are used to unlock Trade Caches in the Crystal Desert and Thunderhead Peaks.“
— In-game description
- Interacting with Sand Piles and Snow Heaps.
- Achieving Map bonus rewards for completing events in core Crystal Desert zones
- You can buy 5 keys per day from most vendors. Vendors within the Domain of Vabbi offer up to 25 keys per day instead.