What are you referring to as "it" here? When OP mentioned getting a gun from "off the street", that's referring to obtaining one illegally, without a provenance chain or any permitting.
If you want to shoot a CEO, its far easier to buy an untraceable gun on the streets (or obtain a non-serialized 80% lower receiver that you drill yourself) rather than an unreliable fully 3D-printed gun.
The implication with this type of argument is that if someone is willing to break the law against murder, they'd be willing/able to break the laws around legally purchasing or owning a gun.
Is it that easy to acquire even illegal firearms in the US, that you can just walk around in NYC to the shadier streets and find randoms willing to sell them to you?
Dude literally sat in a McDonalds with all the evidence on him including the 3D printed gun. The idea of phantom murderers wielding 3D printed weapons is nothing more than a rich guy/CEO anxiety fantasy.
However, you really don't even need to do that. You could just drive across the NY border to a state with looser gun laws, buy one there, shave off the serial number, and bring it back to NY. You could also just steal a gun from one of the many Americans who already own one.
You can also legally buy an unfinished lower receiver in many states (the part of a gun that is typically serialized). Since it's technically unfinished, it doesn't require a serial number. Then you drill a few holes into it and assemble it with off the shelf, also un-serialized gun parts.
You know someone who knows someone.