Lol, it's called trust, and a pain in the ass process. I've been on accounts with well over 20Bil, and I've been on accounts while MMing with 5Bil+.
It takes a certain person to try and scam someone while MMing, and that isn't OMM's. At least not any of the current OMM's.
To safeguard this; I always record account trades where I have to change the password. This guarantees that the password is correct if someone decides to be "funny" and claim it isn't. Of course, I delete the video after the trades complete; I just do it as a precaution. I suggest all MM's do this..