Well, the thing is that I have never witnessed someone leave negative feedback without making multiple attempts to get in contact with the person. If a mistake like your example is made, the seller would then either provide proof of shipment or look for the package when prompted.

On ebay, you can respond to a negative feedback to give your version of what happened and whether or not you issued a refund/made an attempt to correct the transaction. But regardless, a bad experience that warrants negative feedback (especially since people are so reluctant to leave it) cannot be undone... the stress, frustration, and confusion all still occurred.