Question How much can I sell this pc for?

A useful pc, particularly with a GTX1070 card.
Look on your local ebay and find a similar pc,
Filter on completed auctions.
In green you will see what the unit actually sold for.
In the US, GTX1070 sells for $400-$500 by itself.
I have no REAL idea of what the market in AUS is....but to say this.
I watch a fellow down there, Brian with TechYesCity. He is a creator on YouTube (and certainly others) who makes his living buying and selling computers with an emphasis on used "value".
This fellow is confident enough to do what he calls parts hunts, he tells what he paid for the parts he is getting, and still puts them up for sale at a decent mark up. Obviously his time and skill are a valued aspect of that pricing....I gather that working gaming computers hold a decent value there.