It is not said anywhere in the rules, and I've not seen it said. So, when do auctions close automatically if they have no bids? One week? Two? Because technically people can bid on auctions from months ago if they're not closed.
If an auction holder wants their auction closed, they need to have staff close it or at least post in the auction that they want it closed. If they don't I believe it is up to staff to determine if the bid was done within reason. A bid on an auction that is over a year old is clearly not valid, but a bid on an auction that is two weeks old could be valid.
I think it would be cool if you could do something like this: HASH'S FANTASTIC MOCK AUCTION Item: 54 hashhog3000 Heads Starting Bid: 1,000,000 rupees Minimum Bid Increment: 80,000 rupees Auction End Time: 24 hours after last bid Auction Expiration Time: One week of no bids Just the ability to add a time period for which, if there are no bids, the auction automatically expires and all following bids are no longer valid. I'd suggest immediately reporting it afterward so it can be moved from the forums, though.
I've added an expiration time to a few of my auctions. The biggest problem with bidding on old auctions is the seller may not have that product anymore having sold it some other way