But the 49-year-old Rancho Palos Verdes resident is not your average poker player.
He is quite simply one of the world's best.
"He's probably made more money than anyone on Earth in the last five years playing poker," said Mike Sexton, commentator of The Travel Channel's phenomenally successful "World Poker Tour." "He plays in the very biggest games in the world."
For the past 15 years, Greenstein said he has earned seven figures annually playing private high-stakes poker games. He believes he has paid more in income taxes on his winnings than virtually anyone else in the country.
Still, until the last year or so Greenstein was largely unknown to all but elite, deep-pocketed players who play high-stakes home games like those Hustler Casino owner and porn maven Larry Flynt regularly hosts. Bets at the outset of those games can begin at $20,000 and escalate from there.
He eschewed the increasingly popular tournament circuit that now has many events televised. Tournament competitors are driven by ego and publicity, he said. Greenstein has no need for either.
"I don't love playing poker -- it's my job," he said. "The day I don't need to make more money will be the day I stop playing poker, and I'll go on to something else."
Early last year, Greenstein decided that he and his family had enough money. But he didn't go on to something else.
Over his years of poker playing, Greenstein regularly gave money to charity. So, he upped the ante.
He began entering the tournaments he had always avoided in favor of bigger, private paydays.
But this time he had a message.
His goal: to give his tournament winnings to charity. And not just some cash. All of it.
With televised poker surging in popularity, Greenstein's decision came at an opportune time.
Christened with the made-for-television monikers of "Charity" Barry Greenstein and the "Robin Hood of Poker" he has exploited poker's rising visibility -- and the ever larger purses -- to win publicity and cash for the causes he champions.
In round numbers, Greenstein said he has given away a staggering $3 million to charitable causes since April 2003.
"The thing I'm proudest about is my charity work," he said. "I'm using the fact that poker has gotten big to do something positive. I'm hoping people who are wealthier than I will (be inspired to) take some of their money and give it away."
In March, Greenstein gave $60,000 to the Peninsula Education Foundation, which supports a variety of school programs on the Palos Verdes Peninsula, where his two children attend high school. It remains the largest contribution the 25-year-old nonprofit has ever received from a single person, said volunteer President Malcolm Sharp.
In February, Greenstein gave $75,000 to San Pedro-based Rainbow Services, which provides emergency shelter to victims of domestic violence, said Development Manager Linda Alexander.
And in the last 18 months, Greenstein has given $798,000 to the Richmond, Va.-based Children Inc., making him the nonprofit group's single biggest donor, said Chief Executive Officer Marian Cummins. For a 40-year-old organization that supports aid for poverty-stricken children around the world and has an annual budget of about $5 million, the figure represents a significant percentage of its overall operating expenses.
"He just gives and gives and gives," Cummins said. "He's touched people all around the world -- Mexico, Chile, Arizona, Kenya, Bolivia, Guatemala, Lebanon, Colombia, Peru, India, Korea, Kentucky and a whole lot more."
"A lot of people would say, 'Oh, you're accepting money from a gambler,' " she added. "I don't look at him as a gambler. I look at him as a wonderful, intelligent, caring individual with a heart of gold."
Greenstein's example has rubbed off on others; he knows other professional gamblers who now sponsor children.
And if there's a common thread to Greenstein's extraordinary largesse, it's kids.
"If I'm doing in some sense a predatory activity -- like taking money from people -- I need to give back. And I give back to children, because children are often the victims. I usually give up on adults. It's children I think you can change."