Let's confirm that this is a great idea. However, it won't perfectly work in the way op has described it.
I first thought how to send gold by email, and how to set the price of a precious metal in crypto.
As stated above, selling/buying precious metal requires a legal status which may differentiate depends on the located country. Also, i think about why should anybody sell his gold savings in this way. Is this just to get some cryptos?
You can use a trusted person in the forum, everyone sends gold to him and after he is sure of the quailty of gold he tells you to send the money to the second party.
Do you mean hiring a trusted forum member as an escrow? AFAIK, it's not that easy to escrow operations of physical objects in general, and in a project like this, this would require additional cost for the escrow services.