Send the buyer a guilt and shame email

dear xxxxx

thank you for not paying for the item that I sent to you in error and good faith before receiving payment. Despite numerous requests asking for payment of the item you have received I hope you feel very proud of yourself and your dishonest behaviour. Your dishonesty just shows the type of person that you are.  I hope that one day someone can do the same for you. Have a wonderful day.

it may not do any good but you have nothing to lose 🙂