Thanks for all the support and advice from everyone here! I really appreciate it!

I did think about reporting them to ACORN, but I don't have any evidence on my side (as I no longer have the purchase receipt for the items) and the seller is in the US, I'm not sure if ACORN would be able to do much given the situation.... HOWEVER...

After communicating with an eBay staff on Facebook, they admitted they will not release ANY information about the documentation given to them. I asked them that if you can't give me the documentation, at the very least tell me what it is - is it a statutory declaration, a letter from Hermes, or...? Nope. They weren't going to tell me, and they said they will only hand it over when approached by an authority.

And I thought, well thanks, that's extremely unhelpful to a seller and a customer that's been with them for over a decade - so this morning I've filed a complaint with the ACCC, and see whether ACCC would be interested in this. I agree justkidzbiz, there's no way they can legally take my money with no evidence to show me.

I've also faxed (yes, faxed, there's no email contact) to the particular Hermes boutique in Paris that I've bought the item from, asking them for help, to see if they can provide a replacement receipt (it's a big ask, considering this was bought apparently back in 2009), so we'll stay tuned on that front.