The developers of a 47-story luxury apartment tower in the South Loop have put the building back on the market, joining a growing crowd of downtown multifamily landlords eager to cash out. A joint venture of Chicago developer John Murphy and Los Angeles investor CIM Group has hired Newmark to sell the Paragon Chicago, a 500-unit high-rise at 1326 S. Michigan Ave., according to a Newmark marketing brochure for the property. The tower, which opened in 2019, cost about $165 million to build.