If you showed Home Depot sufficient evidence that you intended to lynch someone with it, and it was Home Depot's policy that we sell rope even if it's used for murder, then yes. I think the issue in your example is that you haven't made the corporate policy clear and you haven't made it clear that the employee is convinced of what you're about to do.
> Suppose that they'd already been on trial for it, found guilty and served their time
Ok, so I'm supposing a convicted felon is trying to purchase a gun, which is illegal under 18 U.S.C. § 922(g)(1).
> Do you sell them the gun, knowing that they intend to use it for violence?
No, I do not sell them the gun, because it would be illegal to do so.
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Bottom line, I believe in "innocent until proven guilty." I don't deny services or tools to someone legally able to purchase it because I think they might use it to commit a future crime. I might report them to authorities if my suspicions are backed by evidence, but it's not my job to prevent future crimes. If I work at a glass blowing shop and someone wants me to create a custom bong for them, I wouldn't deny them because they might use it to smoke illegal drugs. Who knows, they might be scientists that need it to conduct a study.
I'm asking if it's unethical to sell someone a gun if you're convinced they're going to do something you believe to be unethical with it. I would say yes. I'm not sure if you would say, "no, it doesn't matter if I know they're going to kill children with it", "maybe, if I only think they're going to kill animals with it", or "yes, I don't think I should sell them the gun if I know they're going to do something I don't approve of with it".
To your second point, we're not talking about "maybe they're going to do something bad", we're saying "here's plenty of documentation that they're going to do something bad".
I don't like this contrived example though, and I suspect you've only set it up so that if/when I say "nope, I won't sell the gun to that person" you'll then say "in this case the gun is GitHub and the felon trying to buy it is ICE" which I do not think is an apt analogy at all.
And would you have the same answer if you believed that law enforcement wouldn't do anything to stop the buyer from doing anything?