I use Australia Post because using a courier comes with hassles of its own.

if your buyer is NOT in a major city, (and thousands of ebay users live in remote areas / country towns etc), then if they are not home when the courier calls they may have to drive 50, 75 even 100km to the nearest courier depot, during business hours, just to get their parcel.

certainly not something a working ebay buyer could manage in a lot of cases.