Complex sentence: The world's most famous paintings, especially old master works done before 1800, are generally owned or held at museums, for viewing for patrons which very rarely sell them, and as such, they are quite literally priceless.
Split into simpler sentences: The world's most famous paintings, especially old master works done before 1800, are generally owned or held at museums, for viewing for patrons. The museums very rarely sell them, and as such, they are quite literally priceless.