I think he is talking about the reletively high number of people that buy and sell gold for cash as private parity sales. Generally, I have been gold authenticated by taking the density with a good scale and water displacement. Though that technically could be faked it would be very hard. If you wanted to do it quickly you could probably cut a corner off because the stuff in circulation is normally high enough grade that it's almost soft. Gold Plated lead would feel off to anyone that's handled gold before from density alone. Gold is nearly twice as dense. That's why it made a great coin in the past. It's far denser than anyother common materials.