PayPal Complaint - Refund fees
PayPal charges refund fees for cancelled transactions. Accordingly, if someone sends money to you by mistake or for a sold-out item, you will be obliged to send full (not partial refunds) and still be charged the refund fees (and the full transaction fees if the buyer is quick in filing dispute)
Moreover, even if your agreement/listing specifies that any cancellation/transaction fees are non-refundable, PayPal does not recognize your agreement or respect your terms and conditions!
I was a target for such misuse. This could be used by many to harm someone's business.