If you issue sanctions, aren't you the one who has to make sure you can enforce them? Like you can say "China can't buy from US-based companies" or "our companies are not allowed to purchase from China".

But this is unilateral, China doesn't give a fuck about your sanctions. You placed them on them, it's your responsibility to make sure they're enforced. "Evading" sanctions my ass they're just doing business as usual, not their problem, not their responsibility.