Update - the 2010 article on Nakano Broadway lives here.
Been living in Tokyo for many years and only recently learnt that Nakano Broadway was another Otaku heaven. Heard much about the place so thought I would go along to see what the fuss was all about.
This is the shopping arcade leading up to the huge Nakano Broadway shopping mall - its just outside Nakano stations North exit (JR Soubu line). Unless you have two red hot steel rods stuck in your eyes, you cant miss it.
Nakano Broadway has 4 floors where the second and third are filled with Audio, darts, adult dvd, army wear, idol posters, figurines and a ton of other otaku gear. Its more likely that you'll find some of that rare stuff that you're looking for here rather than Akihabara.
MANDARAKE is a chain of stores which sell tons of Manga and Figures. MAN is from MANGA (漫画(まんが)) and DARAKE (だらけ) means "lots of" or "all over the gaff".
Many of the MANDARAKE stores sell second hand figures - some of them being very rare indeed. You will find many MANDARAKE stores in Nakano Broadway - each store catering for different areas like robot figures, manga only and what have you.
This is the second floor map and this is the third - notice all the shops with the word "Toy" or "おもちゃ" (toy) in them.
This is a shop with rental cabinets. What you do is rent a shelf in one of these cabinets and pay the shop owner rental fee of X yen per week/month. You then fill it with whatever you want to flog. Each item has a label on it so that people who want to buy your stuff can go to the shop owner and say "I want E-39" which is item 39 in space "E" (just an example). The shop keeper then informs you that you have made a sale and you go and pickup the money at your leisure. Goes without saying that if you dont sell anything, you still have to pay for the cabinet rental space.
Depending on where you go, you will see these filled with Idol, hentai or other rare figures and stuff that people will pay a bomb for.