The Pulse is up a beat or two on the news that more home buyers are signing on the dotted line than were a year ago.

The National Association of Realtors reported today that existing home sales in December increased 5 percent over November’s total. All in all, with 4.26 million deeds changing hands, 2011 was a better year than the previous for sales of pre-loved homes — if only 1.7 percent. It helps, of course, that 30-year-fixed mortgage rates have been hitting record lows about every 96 hours.

The ingredient that’s currently souring the recipe of housing market strength is undoubtedly “inventory.” NAR reports that just 2.38 million existing homes are currently for sale right now. That’s a 6.2-month supply at the current pace of sales, which is the lowest that measurement has been since March 2005. But the problem is “shadow inventory” — the name given to the volume of homes either in the foreclosure process or bank-owned but yet to hit the market. No one knows what that number is, and that uncertainty, among several other macroeconomic factors, is keeping the housing market in the doldrums.

So, when NAR chief economist Lawrence Yun says, “The inventory supply suggests many markets will see prices stabilize or grow moderately in the near future.” Consider taking that with a grain of salt.