I'd like to know more about how sending cash in the mail works in general.  How are the buyer/seller protected by escrow?

There seems like no way for the escrow agent to validate that the buyer of BTC actually sent the cash, unless the escrow agent receives the cash and forwards it to the seller.  This can create a certain counter party risk from the escrow agent.

At least with cash-in-the-bank transactions there is some receipt given by the teller.

It sounds like SaltySpitoon has some experience doing this so I'm curious how fraud is prevented.