The problem with the housing market this spring may not be that inventory is too small.  It also may not be tight lending standards, massive student loans, lingering levels of negative equity or a peripatetic economy.

Maybe it’s just that nobody wants to buy what’s listed.

Inventory trends are closely monitored but nobody keeps track of many ugly ducklings linger amid the two million listings currently on MLSs.  No real research has established that big batch of bad apples are ruining barrel for everybody else.

Yet there has to be an explanation for the strange combination of perky prices, insipid inventories and sad sack sales reported this season.  While hundreds of housing economists shaking their heads, CoreLogic’s Mark Fleming might have the answer.

“Many obsolete homes are now in the inventory as a result of the housing and financial crisis.   Therefore, the inventory of homes for sale that buyer actually want to purchase is even less than what’s on the market now,” Fleming writes in the current issue of CoreLogic’s Market Pulse report.

One result is a “shadow demand” of turned off buyers waiting in the wings until they see something worth buying.

Fleming argues a high ratio of days on market tom all homes in the active inventory is a sign that a portion of the inventory is languishing unsold. In fat, the homes that do sell are snapped up within two thirds of the average time for all homes I inventory.

“I we think that two millions homes on the market is low, increasing housing obsolescence  makes the actual viable housing inventory even less.  Maybe the reason that home sales aren’t increasing is because buyers can’t find anything that they want to buy,” he wrote.