No, crooks do NOT 'order guns online with no background check', and

they don't waltz into gun stores to buy them, either.  Proven again.
The study learned that virtually zero criminals have ever used the internet or gun shows, because that method is easily traceable. It’s much safer for a criminal to acquire firearms on the streets where they’re harder to keep track of, and that’s most criminals method of choice.
And those Evil Assault Weapons?
The vast majority of the inmates used handguns to commit their crimes or protect themselves, very few cited using “military-style assault weapons.” And they said their habit was to get rid of a gun after one year because of the “legal liability” of being caught with a gun that could be linked to crimes they or others committed.”

As for specifics regarding sources for purchasing guns, some of the inmates indicated that gangs have individuals with a Firearm Owners Identification Card who buy guns then sell them to gang members. Others indicated using “corrupt cops” who seize guns then “put them back on the street.”
Crooks get guns illegally; Bloomberg, CSGV and Mommies Want hardest hit