secondhand CD district in Shinjuku?

I heard theres a district with lots of second hand audio CDs in SHinjuku, in the crosshairs of this map:

http://map.yahoo.co.jp/pl?nl=35.41.24.498&el=139.42.8.094&la...

Does anyone know what is the focus, like what kind of CDs they sell, and what the prices are like? Is it a single shop or many different shops in the area?

I don't know if the area is called secondhand CD distict but there're seveal shops near Shinjuku station.
Disk Union is one of the biggest and has a dozen shops, specializing in various music categories, jazz, classic, rock, progressive rock, punk and so on.
Which category are you after?

Disk Union (Japanese)
http://diskunion.net/

Are their prices good, as in lower than retail?

I'm into progressive rock and electronica. Would be fine with exploring other genres too. And are there any shops selling 2nd hand current j-rock CDs?

Book-Off stores around Tokyo. They sell second-hand CDs from like 105 yen to 3000 yen depending on how old the CD is. They Japanese and international music as well as music DVDs too. I think the largest Book-Off is located in Harajuku.