Bought something on classifieds and never received.

[StangFlyer]

My advice to you is:

1) Post this persons username, name, and locale. Make them known to everyone so that no one else will do business with the person unless this is resolved to everyone's satisfaction.
2) As Unit suggested, contact the money order issuer, the postal service, and any other applicable authorities. Get the ball rolling to hopefully put pressure on this person to get your money returned, or at least get your merchandise.

Personally, I would never purchase anything from someone I didn't know online by sending them the money before receiving the merchandise in a fashion that is like giving them cash (i.e. money order, cashiers check, or check). Either request the merchandise be sent COD, or at least pay using PayPal or a credit card where you have some protections through those services. I believe PayPal has refund guarantees and if you don't receive your merchandise after paying with a credit card your credit card company will typically refund your money and go after the seller for the money.