A Storm Lake, Iowa, police sting operation that resulted in the arrest of a felon attempting to buy guns through Facebook demonstrates the latest evolution of a dangerous and unregulated “private sales” gun market in the United States.

In February 1994, the Brady Law went into effect, requiring licensed firearms dealers to perform background checks on customers to keep guns out of the hands of felons and other dangerous individuals. This meant that gun shows -- which attracted unlicensed “private sellers” -- became the ubiquitous public unregulated gun marketplace in the United States. But over the past 20 years private sellers and buyers expanded the unregulated marketplace first through the classified sections of print newspapers before sales went online. Internet sales have since spread from gun-themed marketplaces to popular websites like Reddit, Facebook, and Instagram.

As the unregulated marketplace has expanded, so have the efforts of gun violence prevention advocates to pressure sales venues to enact responsible rules for gun sales. A recently launched campaign by Moms Demand Action for Gun Sense in America has already resulted in more than 55,000 letters asking Facebook to stop facilitating firearms sales. The campaign has been aided by a video explaining how Facebook is used to sell guns that riffs on Facebook's 10th anniversary “look back” feature:

The vast unregulated gun market is a result of the federal law governing firearms sales.

Under federal law, only firearms sellers who are “engaged in the business” of selling guns must acquire a Federal Firearms License and perform criminal background checks on customers. But a vague definition about what it means to be “engaged in the business” allows the private sales market -- where no background check is required -- to flourish. In April 2013, a U.S. Senate proposal to expand background checks to all commercial gun sales failed to withstand a filibuster.

Private sales without a background check are thought to constitute a substantial percentage of gun transfers and a 2004 survey of prison inmates found that nearly 80 percent obtained a firearm though a private transaction without a background check.

The dangerousness of private sales is apparent. Sales without a background check were linked to a 2012 mass shooting in Wisconsin as well as the 1999 Columbine High School massacre. The Bureau of Alcohol, Tobacco, Firearms and Explosives has identified gun shows as “a major venue for illegal trafficking” of firearms, and has specifically linked private sales at gun shows to trafficking operations -- some involving Mexican drug cartels. A 2011 undercover investigation conducted by New York City found that 62 percent of online private sellers agreed to sell a firearm to someone who said they probably couldn't pass a background check. States that require background checks for all gun sales, however, see reductions to violent crime and in particular a lower rate of intimate partner homicides.

According to Nexis, the term “gun show loophole,” a phrase used to describe private sales that occur at gun shows, first appeared in major U.S. newspapers in 1999, with the issue entering the mainstream consciousness after two gunmen took 13 lives and wounded 21 more at Columbine High School in April 1999. While gun violence prevention advocates pushed for legislation in Congress to regulate sales at gun shows, they also recognized that these shows were just one venue where unregulated private sales could occur.

During the late 1990s through the mid-2000s, a coalition of state gun violence prevention groups calling themselves the Campaign to Close the Newspaper Loophole asked newspapers to voluntarily create a classified ad policy to only accept ads from licensed firearms dealers. Like Moms Demand Action's highlighting of the Iowa gun sting, the Campaign to Close the Newspaper Loophole demonstrated the danger of unregulated gun sales through the classified section by highlighting the murderous rampage of white supremacist Ben Smith and the case of a man who used a newspaper ad to acquire the gun he used to kill his estranged wife.

Many papers voluntarily changed their policies. As Diane McFarlin, publisher of the Sarasota Herald-Tribune, explained in 2003, “I believe strongly that this is the right thing to do. We certainly don't want to make it easier for criminals to gain access to weapons.” According to the Campaign to Close the Newspaper Loophole, 76 newspapers with a combined circulation of 8.1 million people changed their advertising policy.

Like newspapers, websites that can facilitate the trade and sale of firearms must decide whether to participate in unregulated gun sales. According to Moms Demand Action, Craigslist, eBay and Google+ all prohibit gun sales. 

In addition to scrutiny directed towards Facebook, in recent weeks the willingness of Reddit and Instagram to facilitate gun sales has drawn media attention. After Mother Jones published an expose detailing how online forum Reddit allows gun sales and has authorized the use of its logo on assault weapons, moderators of the website blocked the Mother Jones story from appearing in Reddit's politics section and users of Reddit's gun subforum designed a 3D printable assault weapon lower receiver that displayed Mother Jones' name and phone number. Instagram, which is owned by Facebook, has also had its underground gun market brought to light by news reports from The Daily Beast, Huffington Post, and The New York Daily News.