[Sangetsu]

The best place to go for second hand cameras is Shinjuku. There are numerous used camera shops within walking distance of Shinjuku Station. It's a little hard to describe where they are, but they are all located around the huge Yodobashi camera/electronics shop near the west exit to Shinjuku Station. Just look for the signs 中古カメラ (used camera). The stores to see are Map Camera, Kitamura Camera, Shinjuku Used Camera, and Camera Box.

There are a few used camera shops around Akihabara, but they have a much smaller selection than the places in Shinjuku. You can get to Shinjuku from Tokyo Station on the Chuo Line. You can get to Akihabara on the Yamanote Line. Both places are close to Tokyo Station.

There are also a few used camera shops in Ginza, but they tend to sell collector items which are rather overpriced. Nikon House is probably the best place to look if you like Nikon stuff.