You cannot legally send fake items through the mail. so because you handed them ( the gold)  in to the Police and they were deemed fake you should be able to get a report from them to give to PayPal who then will take action against the seller ( for the gold)

If PayPal give you grief , tell them your next step is the Ombudsman that usually gets action. Even it it's outside the claim area I would have a go as the thread of both a Police report and the Omb might just jolt PP into action .

To be honest just because something has a Hallmark it means very little if it isn't purchased from a reputable seller and preferably through a Bricks & Mortar store .

Having said that I've bought many a piece of jewellery mainly from Thailand on ebay that is as described. You just learn to sus out the genuine sellers .

I've seen jewellery being 'stamped' right in front of me on Asian markets and passed on as genuine 😄

To buy gold , your only option is to go via the Perth Mint or visit a B & M dealer , there are some decent sellers out there on ebay if your after say a Gold Sovereign etc but for Bullion it's just not worth the risk to buy online.