I don't agree. Selling a gun to a hunter is not the same as selling a gun to a known child murderer. Both is "just" selling a gun but they are very different in practice.
You can't take responsibility for what someone will do with your product. This is insane. If someone buy your car, how can you make sure he will not use for kidnap someone?
Yes. The environment and time is controlled by your employer to maximize productivity for the business. You're free to do whatever you want on personal time, including not working for a company you don't like.
Usually people making your argument also say that private corporations can and should moderate speech. Do you support that? If so, then why do you disagree with workplaces banning topics?
People make dumb things like smoking. We already know that prohibition doesn't work. Putting warnings in the cigarets doesn't work either. So, the next thing would be the seller give a speech about how bad cigarets its for each sale?
Yes you can, and this is why know-your-customer laws exist. This isnt black and white, and depends on the kind of transaction. But if I’m selling cars, and someone comes in looking shifty, and says they want a big SUV to mow people down with, then claiming “I’m just a merchant with no responsibility on end use” is honestly weak bullshit.
If seller offers guns to general public (e.g. gun shop) and child murderer does not have restriction on gun ownership as a part of their sentence, then not selling gun is not a political action, it is an extra-judicial punishment.