Depends on the technology in question. Large companies have compliance offices that actively work to prevent sales to folks for whom it may be illegal. But if they do buy it here and attempt to ship things abroad, that's when the FBI gets involved. Admittedly, it's like trying to hand cup water and plenty invariably gets through, but we do catch some of it. In cases where we know a person got it here when they shouldn't have, the govt goes after the seller/provider and forces them to up their game or face financial penalties.