This sounds like a “process” prosecution to me.
California law exempts police from many of the restrains on firearms that ordinary citizens have.
So officers can purchase firearms not available to other people, with the permission of the Department. Then they can sell the fireams to others, because they are now “used” as I recall.
California firearm law is complex, and at times seems contradictory, so if someone else has more expertise on it, please correct me.
http://www.alloutdoor.com/2014/10/28/ca-gun-owner-guilty-conspiracy-buying-pistol-cop/
Checked with a LEO friend of mine in SoCal. “Off-list” weapons purchased by an officer after the law establishing the approved list went into effect cannot be sold on to a private citizen in the state. The officer can either sell it to a dealer, give/sell it to his department, sell it to another officer or sell it out of state with the usual Fed restrictions but no additional CA ones.
If he sold an offlist weapon to a non-officer, he clearly broke the law.