An example of the ways Mexican criminals sell guns to other criminals:
Merchants make it clear that prices are more accessible if the weapons have been ‘burned’ (used).  A 9 mm ‘clean’ (new) [pistol] is 12,000 pesos [US$934.00], and assault [rifles] like the AK-47 are 15,000 pesos [US$1,168.00].  The latter, like hand grenades, are only available ‘on request.’
In Tepito, those interviewed reported, monthly or bimonthly shipments arrive that are distributed to different destinations. The shipments include revolvers, submachine guns, rifles and grenade launchers, [and] many of them end up in the hands of organized crime groups, they acknowledge.
Naturally these all must be coming from US gun shows. Of course, toward the end of the article, they do note:
A percentage of the weapons, the seller said, come from Mexico via Ministry of Defense personnel who provide [them] in part from weapons seized in raids, or stolen from the ministry’s own arsenal.
You don’t say.