Trader Joe’s plans to open its first store in downtown Philadelphia at 2121 Market St. The 44-year-old gourmet foods grocer expects to open in March 2003 on the first floor of the newly renovated loft-style apartment building, formerly know as the After Six Building. Trader Joe’s will join architectural design company, Daroff Design, in a combined lease of 20,835 square feet of retail space at the location.