A scammer will pay with Paypal funded through a credit card, redeem the bills, and then dispute the charge on the CC. The CC will refund the scammer, and Paypal will refund the CC, take back the money, and charge you a $10 fee.
Or a scammer might pay with a stolen Paypal account and redeem the bills. The actual owner will dispute the charge, and Paypal will refund the owner and take back the money.
Or a scammer might pay with Paypal and redeem the bills, and then dispute the charge, claiming that the account was stolen. Paypal will refund the scammer and take back the money.