I assume it's the snowflake pendant that you got the negative feedback for? There is no mention in the listing of it being a bit tarnished. You've stated it's in good condition with no wear and tear. It would be hard from the photos to tell if there is any tarnishing because they all have a slightly pink tinge to them.

You could try to defend your case, but as there is no mention of the tarnishing in the listing, it's unlikely you'd win. As you sent it to an Australian address, you only need to cover the postage cost from the Australian address that includes tracking. If it has been forwarded onto someone in the UK, the onus is on them to cover the cost from the UK back to Aus with tracking.

It doesn't mean that everything has to be cleaned prior to selling, but if something is tarnished, or scratched, or whatever, you need to mention it in the listing. Yes, a lot of buyers do only look at the title and photos, but if the photos aren't really clear, then it's often hard to see if there is something wrong. If they don't read the description and a fault has been mentioned, they will lose a claim if they are claiming that fault.

Did they open the case through eBay or PayPal? CALL whoever they opened the case with and tell them that you are happy to refund on return of the item, but as it has been sent to the UK, you are not willing to cover the return postage cost as you sent it to an Australian address.