This trick only works with more than one character in your party. Go to a shop and choose an item to sell. Click on it as if you were going to sell it, then change characters to someone else in your party while you still have the item ready to sell. Click on the character inventory to give them the item. After that, click the item again and sell it to the shopkeeper. Stop talking to the shopkeeper and check your inventory. The person that originally owned the item still has it, and if you check the shop you sold the item to, they will also have it. Note: Do not return to the character's inventory that originally held the item while still in the shop; this will crash the game.
Note: This procedure involves editing a game file; create a backup before proceeding. Use a text editor to edit the "rtkrondor.ini" file in the game directory and add the following lines:
Load the game and press ALT + C when the final navigation screen appears. The console prompt with a green dollar sign will appear. Type pyromania followed by ENTER. Press ALT + C again to close the console prompt. Mages now know cast any spell without cost.