I have sent cash - and so far it went ok. I hope it stays that way!
But, it depends on the seller/buyer - like Swanky Mama said, excellent feedback is the key. Also, I would be careful about the amount.
I sent registered, even though the post office does not like that - this way I know whether the other person received their money.
As for concealing cash - obviously no coins, and long time ago someone told me to wrap it in alu foil, bec that way no one could see the money with x-rays?? no idea if that is true or matters, but I wrap cash in foil.
HOwever, there is always risk involved, and I know that there is nothing to protect me, as I send cash. For the seller, I don't see the big deal bec if you haven't signed for anything, you haven't received it.
There was a thread a little while ago by someone who got a cash payment and when she opened the envelope there was no cash and no way to prove it.... so there are some dangers. Maybe you should always open the letter in front of the mailman????