This group is limited to units for sale only; it excludes units both for rent and sale. If a unit was located in a multi-unit structure which was for sale as an entire structure and if the unit was not for rent, it was reported as "held off market." However, if the individual unit was intended to be occupied by the new owner, it was reported as "for sale.
Source: US. Bureau of the Census
Release: Housing Vacancies and Homeownership
US. Bureau of the Census, Housing Inventory Estimate: Vacant Housing Units for Sale for the United States [ESALEUSQ176N], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/ESALEUSQ176N/, October 4, 2015.