Raise your barter level to 10. Once that happens the following items can be sold for more money than it costs to purchase them: Health Potions, Mana Potions, the Town Portal Scroll, and the Identify Scroll. Purchase as many of these as possible (holding down the 'X' button helps on the X360 version) and sell them back to the shop for nearly a 25% profit. This does not work on the larger Health and Mana Potions, only the regular ones.
Item Cloning Trick
When in the town of Torchlight, open the pause menu and return to the title screen to save the game. Reload the same file and then place any items you wish to clone in the shared stash. Now sign out of Xbox Live: This will return you to the title screen (the same effect will be achieved if you return to the Xbox dashboard or turn off your console). Sign in again and reload your file and any items you stashed will also exist in your inventory!