The role of Armslist and other similar online marketplaces as a source of weapons that end up in the hands of criminals on both sides of the border is growing, law enforcement sources told VICE News. While federal regulations require stores and licensed firearms dealers to perform background checks on prospective buyers, Armslist connects users for private transactions that can be done with virtually no questions asked, making it easily exploited for "straw purchases" done on behalf of cartels. 

Thomas Chittum, the ATF's acting deputy director, told VICE News that arms merchants who once had to travel around to gun shows and flea markets can now use sites like Armslist--which hosts classified-style listings for everything from handguns to military-grade firepower like the M249 SAW--to operate remotely and broaden their supply networks.

"By using these online marketplaces, I can do it from the comfort of my home and I can reach a much bigger potential customer base," Chittum said. "That's the appeal of these things. And it's the same appeal that makes Craigslist convenient when you want to sell a couch. This represents a pretty significant source of trafficked firearms."

"It's the same appeal that makes Craigslist convenient when you want to sell a couch. This represents a pretty significant source of trafficked firearms."

Around 70 percent of the illegal firearms seized in Mexico are traced back to U.S. sales, according to the latest ATF data. The clandestine nature of arms trafficking makes it difficult to gauge the true scope of the trade, but Mexican authorities estimate about 2.5 million American guns have poured across the border over the past decade. Mexico saw upwards of 150,000 homicides linked to organized crime from 2006 to 2018, and the homicide rate has remained at historically high levels over the last two years.