Say there is a website allowing users to post ads/classifieds. Every ad includes the poster's contacts details (however, there is no guarantee that they are genuine).
Now, user U posts an ad containing unlawful/objectionable material (for example, offering a house for rent where the applicants are discriminated by race/sex/age etc., or adult photos, or copyright infringing content etc.).
The website owner would prefer to offer maximum freedom to the users and save on content moderation.
Given any of these jurisdictions: US, UK, Canada, AU, NZ, Singapore, are there laws that render the website owner responsible for not taking the U's ad down? Which laws in particular? Can not the responsibility simply stay with the user only, especially that they advertise their contact details?
Would the answer be affected if the site guaranteed that the user's contact details are genuine?