What would prevent gun stores from selling guns to criminals would be the law and the punishments doled out by the government. So if your argument is that Switzerland should punish Swiss companies for dealing with oppressive governments, then yeah, maybe. That's not the topic of discussion, though.

If someone buys a gun in a foreign country, and after returning to his home country he shoots a man, is he subject to the laws of the foreign country? To me that sounds like it would be a breach of the sovereignty of the man's home country. Not that I'm saying this is an apt analogy, but it's certainly closer.