Merketplaces can be held liable for third party sellers merchandise as it’s ultimately up to the marketplace to chose what is allowed and prohibited to be sold.

Imagine a marketplace allowed vendors to sell human slaves and the government tells that marketplace that is very much not okay stop it. If that marketplace continues allowing third party vendors to sell human slaves then that marketplace has now also broken the law along with the third party vendor and potentially even the purchaser depending on how the law was written.