For a instance: having done ebay for along time and dealt with this. These are the rules:
Even if the buyer tells you they will not pay you are bound by contract to honor the sale to that buyer.
You can not send a 2nd chance offer until 1 of these 2 things happen:
Buyer withdraws from sale or you file a NPA and it is finalized. Or if the buyer becomes unregistered and then you can finalize a NPA right away.
Sending a second chance offer before you do this can cause you to get a NPA yourself. if all of a sudden the original buyer pays. Like I said you are bound by contract. It does happen because I have seen it.