WASHINGTON, DC—While sales of new homes in July surpassed all expectations, the volume of trades for existing homes went in the opposite direction. The National Association of Realtors said Wednesday that existing-home sales declined year over year last month for the first time since this past November, with volume of for-sale apartments taking an especially steep fall.

“Severely restrained inventory and the tightening grip it's putting on affordability is the primary culprit for the considerable sales slump throughout much of the country last month,” says Lawrence Yun, chief economist at NAR. “Realtors are reporting diminished buyer traffic because of the scarce number of affordable homes on the market, and the lack of supply is stifling the efforts of many prospective buyers attempting to purchase while mortgage rates hover at historical lows.”

Single-family home sales decreased 2% from the previous month to a seasonally adjusted annual rate of 4.82 million in July from 4.92 million in June. They're now 0.8% under the 4.86 million pace of the year-ago period.