I've bought and sold a few things on ebay, and it never ceases to amaze me how people love to pay to much for things.
For example, I listed an old DVD that had a couple of scratches for auction. It cost me something like $17 new, and I nearly fell over when I found that it had been bid up to $20.... Then a couple of weeks later I saw the same DVD at Big W for $12... It seems that people love winning auctions and will get in a bidding frenzy to do so, paying no regard to the actual value of the item they're bidding on.
It pays to do some research.
If you are buying have a look around at the trading post etc and see what similar items are going for. Work out the maximum amount you are prepared to pay and stick to it - if you miss out there will be other auctions!
Also some sellers put a margin on their postage and packaging costs. It's worth checking the Aust Post website to determine the actual cost, and adjusting your maximum bid accordingly.
If you're selling, do a search for similar items. Have a look at the opening prices they are listed with, and list yours at a much lower price. This attracts bids which starts the frenzy, and the number of bids you get grows exponentially. The opening price is no reflection of what the highest bid will be.
There are actually people on ebay who sell this kind of advice!
Oh and read the rules and help sections before doing anything!