The Huan Long camera market above the Shanghai Railway Station metro is the best camera market in town for second-hand cameras. The most interesting stalls are on the first and second floors, with rare Rolleiflex or Leica cameras from the 1970s (12,500RMB approx); second-hand lenses such as a Canon 70-200mm lens for 1,500RMB (compared to 5,100-17,000RMB for a new one); and a cardboard box full of quirky vintage camera cases (black leather Seagull case, 20RMB). Booth 3051 on the third floor is dedicated to Shanghai Seagull cameras (run by Seagull enthusiast Shi Jian Yu), and you can pick up stunning old and new TLRs for 300-2,000RMB.
UPDATE This market is currently undergoing substantive renovations. There are no stores open on the first floor and higher floors mostly stock new products. For second hand cameras try the top floors of the Xing Guang Photographic Equipment City.