I am sorry to hear about your neg but the way you handled the situation is exactly why ebay have brought in the ebay money back guarantee policy.

There is no doubt the buyer should have taken the time to email you re the missing item but it is only been a few hours since you emailed them so you need to allow sufficient time for them to respond.

If the item has gone missing there is no way the buyer should be held responsible for the postage costs and also if it has gone missing it is up to the sender to lodge a claim with Australia Post - the buyer is unable to do this.