This may have become one of the legendary flea markets in Manhattan, as it has been around for so many years. The market is located in a car garage on 25th street (in the Chelsea area), and is active every Saturday and Sunday, from 9am-5pm. During week days, the place is back to being a garage again.
Being an indoor market, it's a great one to visit at any time of year. The nice thing about Chelsea Flea Market is the diversity of its people—both the traders and the shoppers.
(NB: If you come back at 5pm, chances are that the dealer will give you a better price.)