Gold and silver cannot be counterfeited. An ounce of either can be proofed readily enough, and once a minted coin receives a proof mark from a reputable local agent, it should exchange fairly routinely.
Actually, fake precious metals a already a bit of a problem.
http://blog.jewelrywarehouse.com/2012/06/19/tuesday-jewelry-tips/Nonmagnetic, and with the correct density, hardness, and a thin plating of the real stuff is going to be beyond most peoples skills to detect. There will never be a shortage of greedy sociopaths.
As far as a barter system: in the event of an economic collapse, skills, services, and odd goods will probably be the "coin of the realm".