Hi Paul,
Shipping to Melbourne will be expensive for the buyer due to the cubic, the shipping companies have a formula they work it out with, anyway....
You dont have to worry about the customs and other import duties, the seller has to do that....
The only thing you will have to do is supply an invoice stating the transaction and the goods sold, and they are second hand, I'm not sure about Australia, they may have different laws than we do, but anything we import here with a value over $400 dollars is subject to out Good and Services Tax which is 15%, even if it is second hand.
Most sellers on ebay who do international shipping state that all importation costs and import duties are the responsibility of the buyer, I think that is a good way to clearly state the terms...
Shippers, anyone really, at the end of the day if the gear is packaged properly it desnt really matter who you use, maybe the buyer has a preference, or you could shop around and get quotes
Kevin