William Maxwell is an expert in finance. He’s a professor at Southern Methodist University’s business school, has co-authored a book on high-yield debt and spent years calculating values of financial markets. Yet there’s one valuation he can’t understand: the appraisal of his Dallas home. In August 2010, Mr. Maxwell’s home was appraised at $790,000 as part of a mortgage refinancing. Yet this past spring, when he tried to sell the four-bedroom home for $756,500, the appraisal commissioned by the buyer’s lender, Bank of America Corp., came up with a value of $730,000. Mr. Maxwell said the appraisal killed the sale.