At the beginning of the game, when you reach to the first town of Riverwood start chopping wood and find the man named Hod. He usually stands at the top of the sawmill during the day. He will pay you for "all the wood you can carry" as he puts it. However, there is no limit to how much you can make buy chopping wood and selling it to him. He never runs out of gold like the merchants. Chop and sell wood to him until you have as much gold as desired.