I compare online marketplaces to ISPs: we don't expect ISPs to filter the traffic going over their network to make sure it complies with all laws, because that would be impossible. Having actually run a marketplace in a previous life, it's a similar situation. How is a marketplace with millions of products supposed to vet each individual product? About the only thing a marketplace can do is provide some kind of customer feedback mechanism, make sure customers can get refunds easily and quickly, and kick off merchants and buyers that have too many complaints (though they will probably just register again under a different name).