You've received Item #8. You smash the pot and an object falls into the sand.
You've received the white key!
This will open the white chest that was purchased by Sierra. Once the two of you meet up, the two of you together must agree who opens the chest and holds onto the contents inside (the item maybe passed).
To do so, you must both post permission for that person to open it in your auction threads. Once you both post, it will be opened for only that person.