Start the proceedings. Start a dispute with paypal if that was your method of payment.
Leave negative feedback, put the link to the thread where the transaction started, and then post the entire story in that thread so when others view it they can see what has happend. This doesn't help you but gives you the chance to save someone else.
If you didn't use paypal then the proceedings will be different. Postal money order has been stated that it implies mail fraud if you get "defrauded" but I've never gone through that process. I would start with the post office, get there responce, and then move to the police.
Each step of the way send the seller an account of what actions you are taking thereby giving him/her a chance to make it right as well as the inability to say.. I didn't know.. why me?
Personally.. 140 is absolutely enough for me to spend $500 going after his butt. I guess the amount shouldn't matter but that is enough coin to really upset me. If he's going the dishonest route MAKE the time to go after him with everything you have. Keep very good documentation as the more documentation you have the clearer a picture you can present to the authorities or paypal. If there is any grey area or vagueness there won't be a decision which is the same as "your beat"
Good luck.