This is absurd. Does Walgreens "cause death" every time someone ODs on Tylenol? Or your local Ford dealer when someone gets in a car accident? Smoking killed 6.6X the number of people drugs did in 2017—do you think we should start arresting everyone who owns a 7/11?
There's a lot of people who call for jailing tobacco executives who peddled cigarettes despite knowing about the dangers. I don't agree with those folks, but it's not an "absurd" position to take.
I don't think this analogy works with this scenario. The person selling the gun to a felon is breaking the law but "causing death" would depend on if they knew that was the persons intent. Sort of like if you buy a gun legally and say to the person selling you the gun I plan on killing someone with it, they will refuse to sell it to you and will report you to the police.
I don't think most people intend or want their customers ODing on their product, but it understandably will happen with some given the power of the addiction.
Even still I think where this thread is going is reaching in some respects. Sure people died from the drugs that were sold on the silk road, but from my understanding they wern't sold by Ross directly. Sure it he was getting a profit for the use of the silk road basically being the ebay for illegal stuff, but that to me seems more like organized crime and being a kingpin of sorts.
You're talking about selling things that enable violent crimes against others, while the person you're responding to is taking about selling benign things with which one can only harm oneself. They seem categorically different.
If you knowingly sold a weapon to a felon that is not allowed to posses said weapon, then I would suggest you should be charged with aiding and abetting or even accessory after the fact. While not accusing of "causing death", a punishment fitting the crime should be expected.
It's not the substance which actually matters, but that distributing outside of regulation means you're supporting abuse of that substance in the eyes of the law. You can legally sell morphine to hospitals, firearms to non-felons, or alcohol to adults (add five years for the US).