sausage_1, this has been happening to a number of sellers, without rhyme or reason.

All you can do is edit the listing (which I think you must already have done), and keep an eye on your listings to ensure it doesn't happen sneakily without your knowledge.

You can also phone eBay and express your (polite) confusion and outrage that your listings should have been tampered with in this way, and that you have an absolute right under Australian Consumer Law to set your own price, that no one has the right to change this price or enable an offer (particularly with automatic acceptance of a low price), and that you would like an assurance that this won't happen again.

Record the call with an external recording device, get the CS rep's permission to record, and summarise what's discussed.