Toy Planet is a chain of "recycle" (the most commonly used term in Japan for used goods) shops in Japan. In addition to toys, they sell used books, video games, and other items, mostly for kids in the K-12 crowd.
You can find some very affordable toys - even in the 100 yen range. There are both loose and boxed toys to choose from.
I've been to a couple of their locations. I'd say that in general Toy Planet is geared towards toys that kids can play with, as opposed to, say, Mandarake, which is more geared towards collectors.