A great tip from Steve Nagata:
Flea Markets are becoming quite popular in Japan. In Akihabara, the Mottainai Flea market takes place about once a month outside the UDX building near Akiba station.I found more information on this website:
Here you can find anything from used clothing to Gun-pura to old computers to figures. Sometimes the prices are kind of high, but usually things are reasonable.
Charity MOTTAINAI fleamarkets, a part of the MOTTAINAI Campaign which is set into motion by Prof. Wangari Maathai who is an environmentalist and the 2004 Nobel Peace Prize Winner, focuse on recycling. Bring your old clothes, books, DVDs or videos which you no longer want, or used frying oil to MOTTAINAI station at the fleamarket site, they will recycle them!Might be interesting to visit!
The major sightseeing spot Akihabara has MOTTAINAI Flea market during"Golden Week"* holidays. Apprx 100 venders. Enjoy the market as well as shopping in the electronics quarter. *Consecutive holidays for nearly 10 days from the end of the Apr to the beginning of May.