Actually this would work for PayPal. I believe PayPal doesn't get involved with SNAD disputes and asks you to resolve it between the buyer and seller. However it wouldn't work for a credit card chargeback.
I just checked their policy and it seems you are correct. Wow, that makes PayPal's policies seem really stupid. If I send you an empty box, PayPal won't chargeback. Buf it I can't prove I sent you anything, they will.
Well it's difficult to prove one way or another so that's how they set their policy. Same as receiving virtual goods, they really have no idea if anyone ever got something.
Clearcoin decided to take a neutral stance. If anything goes wrong, all they do is hold the item and return it back to the buyer after a set amount of time. In 95+% of all situations that would work, but someone will always test the limits.