Still Random Chance but their is a way to try and unlock this and all other achievements related to selling the tomes.
Go to Duke at the tavern of any Island and you can buy 2 voyages from him, These are the Key & Chest voyages.
Chest voyage is 30 Doubloons
Key Voyage is 99 Doubloons
These chests & keys are always on that island so you can just keep getting keys and chests as long as you have the doubloons to spare for this and you will get some of the Doubloons back as most items including the Ashen chest can be sold back to Duke.
This is best done when playing on a Galleon with other players willing to chip in for the voyages so this is not expensive for one player.