This data is from the National Association of Realtors. They release a monthly "Existing Home Sales" press release with these statistics. In addition, they have an annual summary spreadsheet for 2007 which I retrieved 2/6/08. Some of the data in this spreadsheet differs from the data in their monthly press release.
The figures for sales of homes are annualized rates for the month indicated (e.g., sales for the month multiplied by 12, I assume). Inventory data is as of that point in time. These are NOT quarterly figures; I simply picked quarter-ending months.
Below is a typical reference for one of their monthly press releases[2] and for the spreadsheet summary.[3]
2008-02-06 07:40 Farcaster 960×720× (19975 bytes) {{Information |Description=Home sales, inventory, and months supply |Source=chart self-made from National Association of Realtor (NAR) public data |Date=February 6, 2008 |Location=Existing Home Sales Chart v1.0 |Author=~~~ |Permission= |other_versions= }}
{{Information |Description={{en|Home sales, inventory, and months supply<br/> Alan Greenspan, the former Chairman of the Federal Reserve, stated:<ref>[http://opinionjournal.com/editorial/feature.html?id=110010981 Greenspan WSJ Op Ed]</ref> ==Source Data