If you have yet to do so, file a buyer protection claim. The link was available in the other forum thread that you posted, but you can also use the below link, select "Shipping on OfferUp" and click on the "buyer protection" option. Make sure to have your transaction ID written down.
There is no ticket number issued, but give it a few business days for a reply.
It is very difficult to prove, unless the seller has a reputation of selling fake coins. The buyer can just feign ignorance or state that it was purchased from "someone else" who also stated that the coin was "real".
The USPS Inspection Service has many competent inspectors, but unless the seller runs a business of selling coins and/or runs a pawnshop, the chance of winning is almost ZERO.