There are a few things that I wouldn't buy on ebay and phones is one of them.

I realise you're not in a position to buy one from a shop. but you could always try the Optus website. They sell phones on there often a bit cheaper than you would get them in store.. The W store also has an online store

I'm pretty sure the other talcos have the option where you can buy online also...

Also, you have a message