That fabled housing recovery? Perhaps it was built on a foundation of sand. The National Association of Realtors announced Tuesday that sales of previously occupied homes rose 2.4% to a seasonally adjusted annual rate of 4.77 million last month, from a downwardly revised pace of 4.66 million in April. But while May marks the third monthly increase this year, those numbers still missed economists' expectations. Sales were expected to rise to an annual pace of 4.81 million units. And selling prices were even worse, tumbling 16.8% from a year ago. The median sales price plunged to $173,000 from $207,900 in the same month last year, but up from $166,600 in April -- and as those of us who took Econ 101 know: unless prices stabilize, there is little hope for a recovery. Shares of home-builders were mixed during morning trading. Centex ( CTX) was down 1% to $8.32, KB Home ( KBH) rose 1% to $13.79, Lennar ( LEN) was off 1% to $7.35 and MDC ( MDC) rose 1% to $29.57.