Just had another guy--I was willing to do a FFL trans out and in to swap a gun with him and he pulled out. If you tell someone you are going to do something, do it.
Let me suggest an alternative to requiring a MO.
I have purchased a few items online, but don't care to send bearer paper out in the mail, especially to someone I don't know. Instead, I have proposed to each seller that I send my check, he negotiates it to insure that it is good, and only then send the item.
This lets him know that he has good funds, and gives me evidence that he was paid. Takes longer, but I've never had anyone reject a deal for it.