step one : have postal employee verify money order is real
step two : cash money order (now you have US currency in hand)
step three : ship package using money you got in step two
step four: buyer reports money order as lost or stolen, it takes a few days to process claim, you endorsed the money order and cash it, you are on the hook for the amount the post office gave you
Like it was said before, people will find a way to try and screw you. I had a guy buy a car for cash and 5 out of 18 of his hundred dollar bills were fake! Luckily we were at the bank, and they kept the fake bills, I kept the title and that guy was extremely pissed!!!