Sales of previously occupied homes plunged more than 27 percent in July to levels not seen since 1995. The National Association of Realtors said the drop was the sharpest in any single month since the group started recording the statistic in 1968. June’s sales were at a seasonally adjusted annual rate of 5.25 million; July’s were 3.83 million. Tax credits for buying a home helped boost sales in the spring, but since the credits expired April 30, sales have struggled. At the current sales rate, it would take more than a year to sell all the homes currently on the market. The median home sold for $182,600, an increase of 0.7 percent since last year.