Home owners who are reluctant to sell because prices have fallen, should do the math, and realize that the market downturn could work in their favor, say practitioners in hard-hit, but still pricey Boston.

Their reasoning may work in many other parts of the country as well.

"People are finding houses at prices they thought they'd never see again," says David W. O'Neil of Century 21 Spindler & O'Neil Associates in suburban Boston.

O’Neil points out to potential sellers that if the house a buyer covets used to be $500,000 but its price has fallen 20 percent to $400,000, it is a deal, even if the buyer’s own home also has lost 20 percent of its value.

In general, the toughest sell is people who bought about four years ago at the height of the market, says Zur Attias of The Attias Group at Barrett & Co. in Concord, Mass. But even for these home owners, selling now may make sense as long as they can at least break even.

He argues that almost everyone forgoes something, and probably several things, that he or she wanted when buying a house. For instance, the home may be in the right school district, but on a busy street. Or it may in a great neighborhood, but it's a Cape, not a Colonial. These are things Attias calls "unchangeables."

He says it’s a good time to sell if a seller can get rid of the most negative unchangeables in his current home, and replace them with better unchangeables in a new home. Once the market really turns around, the growth will be bigger in the better house, he predicts.