To be fair, if you sell someone a gun you can’t always be sure if they’re going to use it to harm someone else. If you sell a gay person a wedding cake however, you can be sure they’re going to have sinful gay sex on top of it.

If they knew that is was going to murder someone they wouldn’t sell it. In theory… The point being that they aren’t, and never have been obligated to sell a firearm to anyone.