The Legality of Commissioning Fan Fic

A friend of mine once said that the reason fanfiction is largely ignored by the authors of original works (and their lawyers) is because of a basic copyright principle. You generally don't get sued if you:

• Don't make money off of someone else's copyright


• Don't do anything that blocks the copyright owner from making money on their copyright

So, here's my question - what about commissioned fic? Groups like the Organization for Transformative Works (the group that runs A03)  argue that fanfiction is completely legal because it is classified as a transformative work (under United States copyright law), and therefore does not infringe on the copyright of the original author - so, in theory, you should be able to sell your fic. However, to the best of my knowledge, there haven't been any major legal cases that have tested this theory.

Since I have a few fic authors on my friend list, I wanted to ask - what do you think? Do you accept commissions? If so, is this something that concerns you?