Antiques expert Khalid has a magpie's eye for spotting valuables to adorn his chandelier-lit riad, much loved by luminaries including Naomi Campbell, Silvio Berlusconi, and Brad Pitt. His gallery is a rambling attic of Islamic art and time-faded Berber carpets, bejeweled chests, and painted cedarwood doors—all with gulp-inducing price tags.