Whenever trading in the classifieds you should have their real name, phone number, email address and mailing address.
In the case of mail fraud, contact the seller with your concern, wait a week, contact with ultimatum, wait a week, then it's under your discretion to contact the authorities and give them all pertinent information regarding the situation.
It is up to the buyer and seller to come to an agreement of exchanging information before a trade, if you're OK with an EMT and mailing address, it's bit like fucking without a condom. If both parties consent, it's kind of hard to cry rape when you get Chlamydia.
That being said, I hope it all works out for you, and that this is a lesson learned with trading online.
Ár skal rísa, sá er annars vill
fé eða fjör hafa. Sjaldan liggjandi úlfur
lær um getur né sofandi maður sigur.