Where to get the Free Trade Quest.
In the town of Bolmus Populi, find the pompous mouse Adolphus sitting outside the tower playing music. He'll tell you he refuses to teach you trading because you're not a mouse, and offer a challenge that you take an apple and turn it into gold.
Where to Trade the Apple and get the Shovel
Take the apple to Capilli Village, enter the library/school and give it to the white bearded teacher. He'll give you a shovel in return.
Where to Trade the Shovel and Get the Lantern
Altar of Mahthildis
Take the shovel to the altar of Mahthildis and head toward the old monastery. before you get there, you'll encounter a carriage and a gravedigger. He'll take the shovel and give you a lantern.
Where to Trade the Lantern for Gold
Take the lantern and fast travel back to Bolmus Populi. Head right out of town to the statue of Magna on the edge of the plains, and fly down to the lowest waterfall. Behind it you'll pass a stone door with a red gem. Keep going to the left and fly downward to the lighted posts, which will reveal a hidden horizontal tunnel. Keep an eye out for the shaft pointing down, and follow it to find the miner. If you arrive without the lantern, he'll offer to buy Iniculus. Arrive with the lantern, and he'll give you a piece of gold from his mine in exchange.
Where to Redeem the Free Trade Quest
Head back to Bolmus Populi and speak to the trader again who will give you 6x strength dusts and the achievement.