4. If a transaction is submitted within 7 blocks but does not become part of the main chain within 1000 blocks the purchase order is canceled. If the transaction becomes part of the main chain after the purchase order cancellation then nastyfans will refund the amount (minus 0.0005 BTC tx fee) to the buyer if the buyer accepts donations. Otherwise it becomes a nastyfans donation. (This is new policy.)
Why the need for the donation distinction? Why not just send back to the sending address? I think taking funds like that could make nasty fans look very bad in the future.
You can not assume that people have access to receive Bitcoin from the sender address. Most people use online services and the sender address belongs to the service not the user.
I would love to send it back. But Bitcoin does not work like that. If there is no donation address then there is no guarantee that the Bitcoin is sent to sender.