Obviously, it's possible that a person sitting in an unfriendly nation, can go onto eBay (USA) and add product listings. And then when they get sales, they can ship the items using the USPS ePacket service from that unfriendly nation. That's the transaction.
In an ideal world, US Customs confiscates it since it prima facie violates our consumer protection laws. However given their budget and priorities, that does not happen. So the items are delivered to the consumer.
But the customer knows perfectly well that eBay is a "flea market", that products should be viewed with skepticism, that this is a 3rd party seller and it's coming ePacket from one overseas country famous for total junk. So this customer is Eyes Wide Open, and thus shares fully in the culpability.
But now, let's add a couple of wrinkles.
Make it a site famous for selling its own quality stock - the site is the seller. And very quietly, this site opens up its storefront to the same 3rd parties from eBay - but the listings look identical - the only difference is one single word in the smallest text on the page.
But still, the shipping time is prominently displayed. Surely the buyer will notice 3-week ePacket shipping from overseas, right?
Except this famous site has not only opened their site, but also their warehousing infrastructure to any 3rd party user for any purpose, even purposes unrelated to sales on the site. Which means those 3rd party sellers can ship illegal junk by the container load to those warehouses, and they ship from inside the United States or Canada.
But Customs can stop a whole container, right? Isn't it blatantly violating the law to warehouse this illegal stuff inside the USA? Nope. Since that warehouse might ship items to Central America or the Caribbean, "Hey, maybe the item is legal there! US Customs doesn't have the right to enforce Costa Rica law!" This makes the warehouse functionally a free trade zone.
On the other hand, this famous site runs their own shipping infrastructure, famously known as "Prime Shipping". Surely if the item states "Ships with Prime", that means it is the company's own legit product, right?
Nope. That shipping is open to 3rd parties also. So those 3rd party items can also ship with prime. The only way to know the difference is that one word in small print on the page.
Can you find it?

Pretend it says "Ships with Prime". I just wasn't logged in, so it didn't know I had Prime.
So what does this mean?
The consumer or a trade agency tries to take someone to task for this illegal activity.
- The consumer said "I am blameless, I bought it on the world's most popular website, and since they sold AND warehoused AND shipped the item On Their Own Trucks, I had no reason to believe the item was illicit."
- The site says "We are not the seller. We are only a marketplace connecting sellers to buyers." (same thing eBay says).
- The site says "We're only a drop-shipping warehouser housing items owned by 3rd parties. We are not responsible for the items."
- The site says "We are only a shipping company, you wouldn't go after FedEx or UPS for the contents of the boxes they ship!"
- The site is arguing that the mistake was the 3rd party seller's, who erred by not un-clicking a checkbox on the interface, which would disable USA sales for that particular item. "A trivial data-entry oversight".
- The seller is beyond the reach of enforcement.