I saw a character turn the hinges on an unbreakable door into lead and simply have the fighter pull it apart. Turning a stone wall into wood then lighting it on fire could be useful too. You could use it to smuggle riches or weapons past guards. Make real swords turn into toy swords, turn diamonds into glass baubles etc.
I once had a character use a similar ability in another game (granted by an item) to turn the item I wanted to steal into looking like a cheap replica. I then called the guards in the museum over to alert them to the fact. The place goes nuts, the curator comes out and opens the cabinet, tosses the 'fake' to the ground and an alert is called to find the real item. I snatch the now much less secure real one.
<edit> In every game I have ever DM'd or played, using magic to affect commerce is a capital crime. Charming a merchant or doing counterfeit etc. It makes sense, otherwise it would be rampant. Given that, you could use this ability to frame another person. You receive some coins from a noble or merchant in public, tell the guards that you think something is not right with the coins, some of the coins are coppers you have transmuted to gold, low and behold, the counterfeit is revealed before the guards very eyes. Who would suspect you? you are the honest citizen that brought the matter before them.