Kmart Corp. is leaving no stone unturned in its efforts to raise cash. In June, it opened its vault of more than 1,000 works of corporate art it had accumulated over the years and put many up for sale. The works include oils, watercolors, photography, posters, prints, tapestries, and sculptures — and even a 15th-Century Chinese Ming Dynasty watercolor on silk. The company has built a makeshift gallery at its Michigan headquarters. It will run six showings per month from June through September to display and sell the pieces.