"There is no greater issue today than what I would call the wealth of liquidity," Zell tells the Counselors of Real Estate midyear meeting. "Today, yesterday and the day before, Saudi Arabia had $350 million more each day to invest than it did a year ago."

"The one conclusion I've come to is, this is a great time to sell properties," Downs says. "No one can know the exact peak of the market, but I don't think we're far from it now."

While Zell would advise against buying a call center or an office building in a remote suburb, he believes the wealth of liquidity could last several years. "An awful lot of our assumptions, perhaps, need to be rethought," he says. For instance, today's relatively low capitalization rates could stay where they are for a decade, Zell says. He notes multifamily complexes traded at 4% cap rates in the 1960s, just as retail malls did in the early 1980s despite 21.5% interest rates. "The classic statement, 'It hasn't been like this before,' isn't true," Zell adds.