Unfortunately you have made a roookie mistake if all your listings are set out the same as the ones I looked at, you have offered Cash on Delivery which does not mean cash on collection. It means that you will arrange for it to be delivered and the buyer pays for the cost of the item and dellivery whern it arrives. Aus Post does provide that sefvice but there is no buyer or seller protection if that serviice is used and it is totally unsafe for both buyer and seller. 

Because this is a service you have offered the buyer is within their rights to sit and wait for it to arriive and there isn't much you can do about it if they do not want to pay for postage and the item up front.

If your current listings all offer COD as a payment method you need to change that to cash on pick up.