Cash purchases made up nearly half of home sales across the nation in September, according to RealtyTrac’s data. AGoldman Sachs study puts the number at 57 percent.

“All-cash is driving up home sales nationwide, which looks good on paper,” RealtyTrac said in its most recentForeclosure News Report.

Cash buyers tend to fall into one of four categories: institutional investors, “rich people,” retirees, and foreign buyers, according to RealtyTrac.

The National Association of Realtors recently reported investors are making up 35 percent of cash deals, and retirees are making up another 12 percent.

“The outsized presence of deep-pocketed Wall Street investors is creating a paradox in the housing recovery,” RealtyTrac said. “A housing boom is taking place alongside a plunging rate of homeownership.”

The high percentage of cash buyers might indicate “financing is too costly,” according to RealtyTrac.

“It’s very difficult for a normal buyer to compete with the corporatization of residential real estate,” Jack McCabe, owner of McCabe Research & Consulting in Deerfield Beach, Florida, told RealtyTrac.

Furthermore, McCabe said, “Once these hedge funds head for the exits, a lot of hedge funds will lose money because of the circumstances they created.”

Institutional investors have spent billions in the housing market in recent years, buying up homes and renting them out. “Leading the Wall Street REO-to-rental pack is Blackstone, the largest private landlord in the U.S., which has spent $7.5 billion on 40,000 houses,” according to RealtyTrac’s Octavio Nuiry.

Foreign buyers are also investing in the U.S. housing market. “For Europeans it’s a no brainer to buy here,” Tilman Otto, a broker with Gibraltar Real Estate Group in Henderson, Nevada, commented to RealtyTrac.

“Prices are cheap and the taxes are low,” Otto said. “I’ve got a couple of clients that have bought five or 10 properties. The prices have nearly doubled since 2010 when they bought. They’re millionaires now—and they’re very happy.”

The current trend of institutional investors and foreign buyers snapping up properties and turning them around for profits is not a sustainable one, according to RealtyTrac.

In fact, the company compares today’s trend to the last housing boom, noting, “Then as now, market watchers shrugged off warnings of an unsustainable housing bubble, caused in part by speculators driving up prices and chasing short-term profits.”

Metros with the greatest percentage of cash purchases in September included Miami, where 69 percent of home purchases were made with all-cash, as well as Tampa, Jacksonville, and Las Vegas, where about 62 percent of purchases were made in cash.