The typical home for sale was on the market for just 21 days in January, according to the National Association of Realtors. That’s down from 43 days one year earlier. It’s yet another sign that fewer homes for sale has caused the homes that are for sale to sell quickly. So quickly, in fact, that 71 percent of the homes that sold in January were on the market for less than a month. Lawrence Yun, NAR’s chief economist, says home-buyer demand has started the year off strong . “Home sales continue to ascend in the first month of the year, as buyers quickly snatched up virtually every new listing coming on the market,” Yun said. “Sales easily could have been even 20 percent higher if there had been more inventory and more choices.” Low inventory continues to be the housing market’s biggest issue. The lack of homes for sale has caused more competition for available listings, higher prices, and fewer options for buyers. According to the NAR, total housing inventory is now at a 1.9-month supply. A six-month supply is considered a healthy market.