Or you can hold the marketplace companies responsible for whoever markets on them.
Of course can't hold foreign marketplaces responsible at all - but that is a different loophole we can close (if it becomes a problem Amazon will ensure it is)
Why should a foreign marketplace get the same benefit of the doubt as a domestic market? Especially if those foreign markets belong to countries that are hostile to mine?
Not easially. Shipping to the US is not hard, and the post office / customs doesn't know what company is sending things. Customs can check of course, but for small value packages they typically do not, so long as the import forms look somewhat reasonable and the import taxes are paid they generally are not looking that close. It is even harder if someone knows they are an illegal company as then they put a false return address on the package and there is no easy way to track them down.
Regular people are not someone custom wants to make an example of for ordering from a banned foreign company. Large companies will carefully check their supply chain to ensure they are not buying goods from banned companies, but tiny companies and regular people won't care as much.
I expect that the EU will have (if they don't already have) rules similar to the US and countries there will generally enforce them. However other countries (China!) will not care as much.
Of course can't hold foreign marketplaces responsible at all - but that is a different loophole we can close (if it becomes a problem Amazon will ensure it is)