It is my understanding that there are two types of paypal transfers...

In one type, a person has the cash transferred from their bank accout to paypal. I believe this type of transaction is not reversible by the buyer unless the seller concurs.

In the second type (this is the one I'm not sure of...) the buyer uses a credit card for payment to paypal, and the "cash" is put into the sellers account. If I'm not mistaken, the buyer who fails to pay their credit card essentially reverses the charge at that time.
I have never had a problem as a seller using paypal to receive funds, since I know paypal charges for the service, and I pay as the seller (the receiver of funds). I have lost unrecoverable money from being a buyer who bought from a deadbeat...I never got money back from either ebay or paypal, and never received the $100 item.

Oh well....