As the housing downturn drags on, plenty of would-be sellers are holding back on staking that “for sale” sign in the front yard.

They’re probably looking for more solid indicators that the market’s freefall has ended. But once they see sales rising and inventory being whittled, they might add their homes to mix–possibly stalling any recovery in home values, says Stan Humphries, real estate Web site Zillow.com’s vice president of data and analytics. This follows concerns that foreclosure moratoria provided a false sense of security that the worst of the foreclosure crisis has passed.

Almost one-third of owners are at least “somewhat likely” to put their homes on the market in the next 12 months, should they see indications of a recovery, according to the site’s first-quarter Homeowner Confidence Survey. That makes for a shadow supply inventory that could help give housing’s recovery an “L” shape–as it bounces along the bottom–instead of a “V” shape, representing an upward climb.

“These sustained supply levels will serve to keep downward pressure on prices, which can only increase as demand begins to outstrip supply,” Humphries notes in a blog posting. “We here at Zillow expect to see a long, drawn-out bottom, with any upward bumps in value tempered by new inventory coming into the market.”

Of course, many of these sellers will themselves be buyers. But, after being burned by the spectacular bust, some will certainly play it safe by renting.