When Justin Olson put his Southwest-style ranch house outside Phoenix on the market, he got what he was expecting: an immediate batch of offers, virtually all above his asking price, which was set intentionally low to attract interest at $197,500. He chose an offer of $210,000.

But then came an unpleasant surprise. An appraiser for the buyer’s bank said the house was worth only $195,000. That limited the amount that the bank would lend, forcing the buyer to come up with more cash or negotiate a lower price.

“There was just no way I was selling that house for less than $200,000,” Olson said. His broker, Brett Barry of Homesmart, advised him that there was little chance of changing the appraiser’s mind. Olson said, “The part that blows me away — the appraisal can be such an arbitrary, personal decision and there is no appeals process.”

Adding to his indignation, a similar house two doors away was appraised and sold for $225,000.

Appraisals are generally ordered by banks so they can verify the value of collateral before granting a mortgage. Before the housing crash, when home values seemed only to rise, appraisals were almost an afterthought.

But now, with banks far more cautious about lending, a low appraisal can torpedo a deal.

The problem is so widespread that this week the National Association of Realtors blamed faulty appraisals for holding back the housing recovery, saying its members had reported that more than a third of all deals were canceled, delayed or renegotiated to a lower price because of a low appraisal. Several real estate agents said they were starting to include appraisal contingencies in their contracts, spelling out how much a buyer would be willing to pay in cash if the appraisal fell short.

Appraisers use previous sales of comparable houses to help value a home. If prices are just starting to climb, and sales take two or three months to close, there can be a lag before the change in prices is observed.

The Realtors report said appraisers were improperly using foreclosures and neglected properties as comparable homes, failing to account for market conditions like scarce inventory and bidding wars, and working in areas where they lack local expertise. The report faulted banks for using inexperienced appraisers and for creating unrealistic requirements, like six comparable sales instead of three, at a time of few sales.

“It’s holding sellers off the market,” said Jed Smith, the managing director of quantitative research for the Realtors group. “Sales volume could probably be an additional 10 to 15 percent higher if we had normal lending practices and if we had normal appraisal practices.” That in turn, he said, would create more jobs.

Appraisers and real estate brokers agreed that a ban, imposed since the housing crash, on loan originators’ handpicking appraisers had led to the use of appraisal management companies that take a healthy cut of the consumer’s fee and hire inexperienced, low-cost appraisers.

But appraisers took issue with the complaints and pointed out that unlike real estate agents, they have no bias or incentive to help complete a deal.

“Appraisers don’t set the market; they reflect what’s happening in the market,” said Ken Chitester, a spokesman for the Appraisal Institute, a professional association. “So don’t shoot the messenger. Blaming the appraiser for a bad housing market is like blaming the weatherman because you don’t like the weather.”

Olson and his buyer compromised on a price of $205,000, less than initially offered and therefore, some might say, less than the house was worth.

But any transaction involving a mortgage is limited by the appraisal — an assessment that is part science, part art and is based on a variety of factors like location and square footage.

Though Olson’s house was in good condition, the house nearby that sold for more had at least $30,000 worth of upgrades, said Craig Young, the broker who represented the seller. But Young said appraisals could still be unpredictable, pointing out that a home across the street sold for even more, $239,000.

Some appraisers said agents misunderstand the way homes are valued. For example, although bank-owned homes generally sell at a discount, that is not true in every neighborhood, said Dan McKinnon, who runs an appraisal company with his wife in Phoenix. Appraisers, therefore, do not automatically make adjustments if they are using such sales for comparison. Some bank-owned homes are in good condition, and in some neighborhoods bank-owned sales dominate the market and thus determine prices.

“If that property is in similar condition to your subject, it is direct competition,” McKinnon said.