I'm a little confused here. You said the second "scammer" paid you the money for the Bitcoins and never received his Bitcoins. If you're taking his money and not sending him the Bitcoins, does that not make you a scammer instead?
Not if this is done to scam the scammer. The moment the scammer tries to reverse the transaction, you know he is trying to scam you. You rely on two things happening with this, which would be that PayPal would reverse the transaction and that the zero fee transaction would not be picked up by a miner, doing a good deed for the day. ^hmmmm^
I would not recommend people to try this, because it can back fire and you could lose a lot of money. Just buy Bitcoins on a exchange or Localbitcoins.