The Chapman Brothers have been buying, stealing, and defacing Hitler's art since 2008. I think they may have at one point even snuck into a gallery and painted over something before anyone could catch them. Art dealers are now doing whatever they can to avoid them any time one of Hitler's pieces goes up for sale, so they've resorted to using 3rd. parties and pseudonyms to acquire his stuff.
They routinely get death threats from neo-nazis, of course.