You could send them an email signed with the private key that you originally paid with (Bitcoin has a message signature feature in the GUI). Edit: see here: ht

You could send them an email signed with the private key that you originally paid with (Bitcoin has a message signature feature in the GUI). Edit: see here: https://bitcointalk.org/index.php?topic=223713.0 https://bitcointalk.org/index.php?topic=223713.0