Unless you know you are dealing with a reputable person there is no safe way. A thief is a thief and they don't care about contracts, USPS money orders or anything else. They always believe they are going to get away with it so those types of things are not deterrents, only tools to make you feel safe while you are getting ripped off.
Or have I became too cynical in my middle age?
I will not trade with anyone that I do not personally know or that does not have a stellar reputation in the industry. If the person does not meet those two requirements then they must ship first. End of story. I have never been ripped off on a trade because I stick to those three rules.