i guess if you run into the wrong person you will have a problem.i had a paypal service transaction and the guy claimed he never did the order.fortunately i had all the emails that corresponded to the transaction and i was able to foward them to the paypal investigative center.
they emailed me back and stated that they agreed the customer lied and did initiate and recieve the service,but stated his credit card company still refused payment therefore paypal couldnt back up the transaction.
i have never used paypal again