If it were me, I would contact them once again and give them all the details ( address, phone number etc ) say that you would like contact within 24-36 hrs and see how you go. If they don't respond, then I would then take it to the next step. They ( buyers ) may have had something pop up or just haven't been at the computer. It is frustrating, but things do happen unexpectedly. BTW have you read their Feedback etc. Are they new to this ?  If you go through FB you may discover they are " players" or just dizzy. If they are Players, I would just open a case against them. That may do the trick. 🙂