I have watched various Qantas Club pass auctions on Ebay for quite a while now.
There are three main types:
1. A person who got offered one with their Silver membership, Amex Card, or as a recovery from Qantas who offers it for sale.
2. The "4 pack"/"2 pack" sellers. These have no expiry date on the front of the card in the pictures on Ebay.
3. The "Member's Guest Pass" sellers.
My comments on each.
Seller type #1. Normally a legitimate card - just being sold as excess to the person's needs. Note that new Qantas Club passes have "Not for sale" clauses on them - although I don't think Qantas can track that. They also should have an expiry date that the customer should put in the auction description. Newer cards have a holographic (cheap looking!) sticker on the front of the card.
Seller type #2. I've always been suss about these cards, but they do appear legitimate on the surface. The card should have an expiry date on the front, (such as Valid Till End March 2008), but the auctions often say no expiry. I suspect that they may have been obtained in unusual circumstances based on the number of cards being sold and the length of times these auctions have been around for. It is also possible that they are offered to some Corporate Customers and or Travel Agents based on sales, rewards or to help convince them to fly with Qantas.
Seller type #3. These cards are most likely worthless, unless you are a Qantas memorabilia collector. "Member's Guest passes" were phased out in 2005 (IIRC), and I'm not aware of any re-issues of these cards. Luckily the last batch of these Member's Guest cards were expiring either late last year or early this year, so unlikely to be sold much these days.
What is a pass worth? All that depends on what you are willing to pay and what it is worth to you. I would value a Qantas Club visit around $40 for an International Journey, and around $20 for a domestic journey. It all depends on whether you drink alcohol, whether the bar is open when you visit, how long you are at the airport for, whether you want to use a computer etc etc etc. The real value is variable based on how you use the card.
The physical cards themselves seem to go for between $20 and $100 each on Ebay depending on what people are willing to bid...
There is also a wealth of discussion on this topic already here in various threads. Have a search for "Invitation" or "ebay"