Note: this page, well over a decade old, may be removed shortly.
Wayne H. suggested:
First, put a STOP notice on your money order [or check]; even if it was cashed, notify the issuing company of the problem and do it anyway. [If you used a credit card, notify the credit card company - note that most have brief time limits for this. Using a credit card is often helpful because it is easier to stop a credit card than a check or money order, you can do it up to 30-60 days after the sale, and they may help you to resolve the problem].
[Webmaster suggests using your state's consumer affairs department, though they vary in quality and power].
If you sent this check or money order through the U.S. Post Office, you can report it as a possible mail fraud case. Which of course means again going through everything you did above, but usually, they would move the fastest, especially if there have been previous complaints against this person.
I would recommend doing all of this in steps, giving some time to the vendor to respond to each before you go to the next step; at least a week between each. Don't threaten him personally, just state calmly of your intentions before doing them, follow through if you say you will, and keep copies of all correspondence between you two, and anyone else acting on your behalf. Any copies you send out, make sure you can keep track of and never give out the originals! A notarized copy is as good as the original document.