This is the market Boesky has specialized in; she has helped build markets for some of the most sought-after artists working today: Murakami, Furnas, Yoshitomo Nara, Lisa Yuskavage, and Sarah Sze. She now has 21 artists, a new building, and big plans. But the art world is full of predators, and a pack of them are aiming to take her down.

“People are really going to be letting off some steam tonight,” Boesky says, as she steps into a puddle on 10th Avenue. Her black suede heels and the hem of her gray skirt get spattered, but she doesn’t seem to notice. Boesky, now 40, is stylish but not overly concerned about looking polished. Her legs are bare, wisps of blond hair have broken free from her ponytail, and a green ring tattoo decorates her fourth left toe, a vestige of a postcollege trip to Borneo. She is heading to Phillips de Pury, the downtown auction house, for the last sale of what has been a wild run of auctions: The previous week, Sotheby’s sold $239 million of Impressionist and Modern art. The next night, Christie’s sold half a billion dollars’ worth in two and a half hours.

As Boesky enters a rowdy auction room, the chatter ranges from gossip about the week’s results to speculation on which pieces would sell that night and for how much. “How’s Barnaby doing?” several people ask Boesky.

Two nights before, at Sotheby’s contemporary-art auction, Ovitz sold Heartbreak Ridge for $520,000. It was a strong sale—Ovitz had bought it for $12,000—but many thought it would go for $1 million. Boesky knows that nothing can be more damaging to a young career than bad post-sale buzz, so she immediately gets to work burnishing Furnas’ image. “He’s doing great,” she tells a collector, smiling. “I mean, he’s 33 years old, and his painting sells for half a million dollars. How many 33-year-olds can say that?”

A few weeks later, in a small booth on the floor of the Miami Beach Convention Center, Boesky is having an extraordinary morning: In two hours she has sold nearly everything she brought down here. It’s the first day of Art Basel Miami Beach, one of the biggest art fairs in the country, and the money is flowing fast. Just about every major collector is here, along with a slew of beautiful people: Peter Brant and Stephanie Seymour, Jay-Z and Beyoncé.

The competition among dealers is cutthroat. Hundreds of them from all over the world fight to catch the attention—and the dollars—of browsing collectors and curators. A few weeks before, Boesky says, the fair’s administrators had threatened to give her booth to another dealer. She argued her way back by promising to deliver what savvy collectors crave most: new, raw talent—Jay Heikes, Ivette Zighelboim, Yi Chen. It pays off: Her booth generates big buzz and lures influential collectors such as Don Rubell, David Mugrabi, and David Teiger.

At one point during the fair, two of Boesky’s competitors stop by to say hello. They smile and admire her wares, but as soon as they are out of earshot, they begin whispering about her. “Did you know Murakami is leaving her?” one says. “That’s going to hurt her business. I mean, I kind of feel sorry for her.” The other looks around furtively and then, as though revealing a major secret, says in a hushed tone, “There’s something else: Did you know her father went to jail?”

Almost two decades before Enron, Ivan Boesky was the world’s most famous corporate crook. Until her father was accused of insider trading in 1986, Marianne lived a charmed life. Her family was tight-knit and rich-rich. She grew up on a 200-acre estate in Bedford, New York. She used to watch her father play tennis with Carl Icahn. She lived in a mansion filled with art by Monet, Degas, and Giacometti.

After her father was busted, Marianne—then a sophomore at Duke University—saw her ideal life vaporize. Her parents divorced; a brother changed his last name; someone set fire to her car. All her father’s money was gone, chewed up by fines, shareholder lawsuits, and a divorce settlement.

Despite it all, she still loves her father. “I learned a lot watching him,” she says. “My father’s motto was ‘Don’t measure yourself by your things; they could be gone tomorrow.’ ”