As a seller, your best packaging material is the padded bags available in mulitple sizes from Aussie Post. They already have the bubble wrap inside them so for most items thats all the padding you need. I find sizes 2 and 5 will fit almost everything.
Buy them in lots of 10 (pay for 9) or 100 (approx 40% off) for extra discounts.
I have found that everything except glassware and breakables travel fine in these bags. Just pop the item into a zip lock bag and into the padded bag and away you go. Yes breakables will need to go into a box, just remember not to pack the padding too tight or you will break the object inside - shredded paper goes a long way in this instance.
The second most useful thing about these bags is you pay postage by weight only! No cubics to measure as they're NOT a square. Get good quality digital scales that weigh up to 5kg (I bought mine on eBay) and you can work out exactly how much your item will cost to send. Your local post office can give you a Post Charges booklet or you can look the prices up on the net. I prefer to use the book as its quicker. It also has international rates, registered post and express post charges.
Remember anything up to 500g is the same price to post anywhere in Australia.
In the long run, if you're worried about under-charging postage, don't list the item with postal charges and just let the winner know how much when the auction finishes.
Remember also to offer postal discounts to winners of multiple auctions - that way you get a better chance of having more sales and higher sales as one bidder may try for more items.