Don't ship the item until you take the USPS money order directly to the Post Office. They will inspect it, check it for legitimacy, run the number against a list of known stolen checks, and then cash it for you.
It is important that you take it to the Post Office directly and not your bank if you want to know if its legit right away.
If the buyer INSISTS on a USPS money order I would be skeptical, but if YOU initiate the request I would feel quite confortable.
One caveat, I have heard of rare instances where even the Post Office was fooled but I feel this is probably the closest to 100% guarantee that you are going to find other than cash, obviously.
PS. Money orders purchased from 7-11, Quick Check etc. are not the same thing as USPS money orders. Use extreme caution accepting those.