It seems California has an "approved roster" like Mass does, and cops there have figured out a way to profit from it:
http://www.latimes.com/local/lanow/la-me-ln-illegal-gun-sales-20170412-story.html
Law enforcement officers may be illegally selling guns, ATF says
The head of the ATF’s office in Los Angeles has sent a memo to Southern California police chiefs and sheriffs saying the agency has found law enforcement officers buying and reselling guns in what could be a violation of federal firearms laws.
The memo from Eric Harden, the federal Bureau of Alcohol, Tobacco and Firearms’ Los Angeles Field Division special agent in charge, describes the finding as an “emerging problem” and expresses concern about “the growing trend of law enforcement officials engaging in the business of unlicensed firearms dealing.”
He did not say how many officers the agency has found purchasing and reselling weapons, but the memo — dated March 31 — says some officers had bought more than 100 firearms. Some of the guns have been recovered at crime scenes.
But Harden wrote that the goal is “to educate, not investigate, to ensure law enforcement officials comply with federal law in order to avoid unnecessary public embarrassment to themselves and your department/agency.”
http://www.latimes.com/local/lanow/la-me-ln-illegal-gun-sales-20170412-story.html
His memo focuses on the purchase and resale of “off roster” firearms. Those are guns that are not on an approved list of weapons that can be sold to the public.
The California law establishing the roster has an exemption that allows sworn peace officers to purchase such weapons, and an additional one that allows officers to resell the guns under certain conditions. But if officers are buying and reselling weapons for profit as a business, they need a federal firearms license, or FFL.