A landmark Banksy is hitting the auction block via Fair Warning , carrying one of the artist’s loftiest estimates yet. On May 20, Girl and Balloon on Found Landscape (2012) will be sold in a rare live auction by the members-only online sales platform at Tiffany & Co.’s flagship store in New York. Consigned by a private collector, the work has never been seen in public before and is tagged with an ambitious $13 million–$18 million estimate. “It is becoming clear that Banksy is probably one of the most consequential artists of our time,” Fair Warning founder Loïc Gouzer said in a phone call. “It’s hard since within the art world, you’re not supposed to like Banksy because the art world didn’t make him—he’s the only artist who made himself.” Gouzer, a former Christie’s rainmaker who helped lock in the $450 million Salvator Mundi sale, is considered a bit of a market maverick . He believes Banksy is on a similar trajectory to Jean-Michel Basquiat . “That’s why I decided to do it,” he said of this sale.…