I've done a bit of selling on Ebay too and Turbo has pretty well covered the points to note. With bulky or heavy items I usually go to the Aus Post web site and work out sending to country WA from here (Cootamundra), and put that as a postage example in my item description, noting that it would be less for shorter distances.
Pack things well, Aus post could break an Anvil
Cheers
Bill