As of now 6/28/11 this achievement is slightly bugged. You must sell 100,000 worth of items in a single session not adventure as the achievement says. While this seems like a hard task its really quite simple. The achievement can be earned at any point in the game where you have even a single item to sell for gold, obviously the more items you have and the more expensive they are the less time it will take.
Anytime you sell an item it will be added to the buyback menu in all the shops up to a maximum of 42 items with the most recent 42 items taking precedence. The trick is that you can buyback an item for the same exact price you sell it meaning you can buy and sell the same item or items indefinitely and the gold spent counter will keep increasing. Just make sure you do this all in one session without exiting and you'll have the achievement.
Don't worry if you get the deed before the achievement because the deed tracks all items sold across all sessions on the same character it's just the achievement tracker that is bugged.