Instructions
Click the icon representing the Sim holding the treasure you want to sell.
Click the "Inventory" tab in the bottom part of the screen.
Hold the left mouse button on the treasure you want to sell. When you hold the item, the "Sell" box beside it tells you how many simoleons you will receive for selling the item.
Drag the item into the "Sell" box and release the mouse button to sell the item. Your Sim immediately receives the simoleons in his account.