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.
Well, fraud can be prevented a few ways. Registered mail is the best option, because its the safest shipping wise, and can be insured. When doing registered mail, the post office employee has to physically watch you seal up what you say you are sealing up in the package, so you can't commit insurance fraud. If you are shipping via registered mail, you can't just ship someone a wad of construction paper, with a tracking number and say it was cash, because the post office wont allow you to do so.
I've done a few smaller cash trades which didn't require registered mail, and we just did overnight shipping, but for larger sums of money, it is certainly worth doing everything right.