There are two types that are often for sale through EBay. The old Qantas Club Member's Guest Passes, and the Complementary Invitations.
The Member's Guest Passes are useless, even if they have an expiry date that has not been reached. Qantas changed the entry conditions a while back making these passes redundant and now worthless.
The Complementary Invitations are given to members when they achieve Silver Frequent Flyer status. These invitations are transferable and come up for sale every now and then as the member who received it is not going to use it. They tend to sell for around $40 on eBay, but the price does vary significantly as with most ebay auctions.
Its interesting that some are currently showing over 22 pounds, and yet the same seller has some up for buy-it-now for 7.50 pounds. Some buyers just do not do their research first.
The Complementary Invitations are the only ones I would every consider buying if I needed access. However, the seller of the ones linked above has so many of these for sale that they certainly did not obtain them by achieving Silver FF status. So that does tend to raise some questions about how they came across so many in a legitimate manner. As always, its a matter of buyer beware.
Be aware that you can only use them at real Qantas Club lounges, not partner lounges.
If your planned trip is long enough and you chose the right booking class, flight numbers and routing, you can achieve OneWorld Sapphire status through the AA Platinum Challenge and hence get lounge access any time you fly OneWorld airlines (except for AA domestic flights). For many people this is more economical than purchasing Qantas Club membership. It can also bring benefits in better burn rates for FF awards.