You can buy a return label using Shippo. It is a bit convoluted. You only need tracking. It might be advisable to pay extra for signature confirmation because you wouldn't want a $600 package left unattended at your door.
As you already stated, there is a mechanism for the buyer to provide you with a quote. My preference would be to use Shippo, because you can ensure you get the service you want and that all the customs stuff is done properly. It can be a big headache if the buyer fills out the customs form incorrectly, and you get hit with an import charge on a buyer return. Don't use UPS or Fedex, only use USPS.
You should be sure that the buyer opens a return before you do anything. Provide the label. Ensure the chain is the same one you sent. Then refund.
I understand the buyer being upset if they spent $600 on what turned out to be a counterfeit item, but one thing to consider is that gold is likely a high risk item to sell. One tactic scammers use is to scare and intimidate sellers, along with hoping they don't understand how eBay works. For example, a lot of less experienced sellers don't understand that they HAVE to provide a label before the deadline or else the buyer can get a refund and keep the item. Assuming this buyer is being abusive, it would raise a red flag. Given the issue, it's possible that they are just upset and have bad impulse control - but I absolutely would be very cautious with the return.
If they are scamming you, after you provide the label, they will probably try to intimidate you into refunding them without a return.
In terms of the negative feedback, while you didn't know the item was not gold, if it turns out that it was actually counterfeit, their feedback is not out of line. We're still responsible for what we sell, even if we make a mistake. I would simply reply with something simple and professional stating that you do your best to ensure that everything you sell is accurately described. The buyer was provided a return label free of charge, and a full refund pending the return of the item.
If you send the buyer a return label and they opt not to return it, you should be able to get the feedback removed.
My only long term advice would be unless you're running a full time business where you're an expert and fully immersed in the field, I would avoid high risk items like gold or jewelry. I have no idea what dealers pay, but you should weigh the amount you'd take home on eBay after fees+expenses vs what a dealer might pay you and decide if eBay is the best option when considering the liability you take on for something like gold.
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