When buying material things, there is recorded delivery, posting receipt that can be used as evidence for the dispute by the seller. There is no evidence left to show paypal that you have sent your money, however even if there was, i think buying btc is against paypal rules aniway so..

I would say the safest way would be to open an account in a currency exchange (mt gox, btc-e, etc) and buy/sell from there since they will intermediate the exchange and you have no contact with the buyer or seller.