You are wrong either knowingly or unknowingly, paypal is reversible it is not a fact how you are sending (fnf or service). The reversible might happen for several reason. Such as--
1. Hack paypal account
2. Stolen credit card
3. Report via credit card issuer
4. Report via bank (now it cost 20$ additional from the receiver. That means for this 300$ transaction receiver will loss 320$)
5 There might have many more----------