In Buddhist ethics, where does personal responsibility end?
MN 61 is the most basic sutta about ethics, which refers to not harming oneself or another.
SN 55.7 is more extensive, as follows:

Reflecting this way, first, he gives up killing beings. Second, he
encourages others to abstain from killing beings, and third, he
praises the act of abstaining from killing beings. So, his bodily
behaviour is purified in three ways.

Therefore, not engaged in the wrong livelihood of selling weapons, poisons, etc, is an example of: &quot;encourages others to abstain from killing beings&quot;.
Then there is AN 4.62 similar to DN 31), which extolls the virtue of blamlessless in relation to livelihood. If you sell weapons then learn your weapons were used by others to kill people, you might feel blame.