The newest problem for the slowly improving housing market isn’t a shortage of serious buyers, it’s a shortage of good homes. Would-be buyers are packing…

Would-be buyers are packing open houses and scrambling to make offers on properties before they are even listed. Bidding wars are erupting. And real-estate agents are vying fiercely to represent the few sellers that do exist.

The number of American homes with a “for sale” sign hit 2.5 million in April, the lowest for an April since 2006, according to the National Association of Realtors.

The sharp drop in inventory and rock-bottom interest rates have helped stabilize even some of the hardest-hit markets, including Southern California, Las Vegas, Phoenix and Miami.

Some real-estate professionals are concerned the lack of inventory might turn off potential buyers, stifling the recent rise in home sales.

The much-predicted foreclosure wave that was expected to dump more homes onto the market has not materialized. Fewer borrowers are entering default, and banks are better managing the properties on their books.

In addition, professional investors bankrolled by private-equity firms and hedge funds are pouncing on bank-owned homes, often turning them into rentals.

A dearth of new construction also is constraining supply.

In April, the most recent month for which figures are available, the number of completed single-family homes available stood at 46,000, the lowest level since the Census Bureau began keeping track in 1973.

Some 70,000 were under construction, also near historic lows.

The inventory problem has been exacerbated by the plunge in home prices since the go-go years.

Many people who bought at the top of the cycle are so deeply underwater that they can’t get the price they need to sell and therefore don’t put their homes on the market.

Negative equity hurts

“We know negative equity holds back home sales, but it also holds back the listing of sales,” said Sam Khater, an economist with CoreLogic, a company that tracks the mortgage market.

“Today it is holding the market back.”

The lack of available homes is maddening for those consumers who thought 2012 would be the year to buy.

Alex Gruenberg and his wife, Kristina, both 27, lost out on a home that ended up going for $30,000 more than they offered.

The recently married couple have new jobs in the Los Angeles area and are looking for a pedestrian-friendly neighborhood with decent dining options.

They are now trying to find homes before they are listed.

“We are really learning that there is sort of an inside element to that,” Gruenberg said. “Things are going in days.”

Glenn Kelman, CEO of Seattle-based real-estate website Redfin, said the recovery remains tentative but the market has grown competitive because sellers believe they have time on their side, while buyers have a sense of urgency given low interest rates and relatively cheap prices compared with the bubble years.

“It is a precarious situation, but the real issue is that nobody wants to sell a house right now,” Kelman said. “So now we have classes for our real-estate agents on how to win a bidding war.”

*the Seattle Times