SURUGAYA Akihabara Station Front Branch is something of a hidden gem to me. Sometimes, you can find rare things here for pretty cheap. They deal in both used and new goods, too.
The mystery ten CD sets they sell for 1000-2000 yen in the storefront are really fascinating!
It might be an interesting venture if you're looking for a new genre to get into.
There's also super rare graphic art framing the staircase between the first and second floors! Is this a subculture museum?!