So we all know (except the OP and the seller) what should have happened.    

1.   The seller lists an item at $80 with best offer - free postage.   The seller has best offer parameters set at say $60 minimum, so that he will not be bothered by unreasonable offers.

2.    The buyer makes an offer $40.

3.     The seller should have made a counter-offer saying I will accept $40 if you pay the postage $13.50;   which would have been very reasonable in my opinion - I think to make a 50% offer is a bit rude actually.

4.   Then the buyer could have accepted the counter-offer or declined.

This did not happen, probably because the seller is very new and did not know he could do that.

5.    The buyer gets an invoice for $53.50 which he does not want to pay.   So he immediately cancels the transaction - which I understand under the new rules you can do if it is within the hour.

This did not happen either.

Now, in reality, the buyer does not want to pay $53.50.  The seller does not want to sell a $80 item for $40 which includes $13.50 postage.   So the buyer can ask the seller to send a cancellation request, the seller gets their final fees back and re-lists the item.   The buyer looks elsewhere for a cheaper bag.