The Lost Trader side quest is a rescue mission, in which a trader asks you to save his brother and return him safely for a reward.
The next Fable Anniversary walkthrough explains what to do during the quest and how it connects with another side quest that you may have started earlier in the game.
Money: 2600 + 110
Renown: 150 + 214
After you get the quest card you need to teleport to Oakvale and then take the exit to the Clifftop Path. The actual quest starts after you speak with the trader in the Abandoned Road area.
He tells you about his lost brother, who is at the other end of the area. However, the area is filled with bandits, which should not be a problem for you.
Kill all of them until you reach the traders brother (watch the red dots on the mini map). The brother follows you automatically, so dont rush back because more bandits will appear on the road.
Deal with them carefully so you dont hurt the traders brother. Make your way back to the trader to collect your reward and complete the quest.
Apart from your reward he gives you Treasure Clue 2, which is an important item in the Hidden Booty Hunt side quest.