I am currently living in Kawaski but moving to Minato-ku, Tokyo, so I found something useful to sell for when I'm moving.
If you have things you don't need but have to sell, there are some useful websites, groups and apps in Japan.
1. Mercari
http://mercari.jp
We used Yahoo auction but nowadays, Mecari has become more popular than Yahoo auction.
2. Amazon
http://www.amazon.co.jp
It is especially useful for selling books.
3. Tokyo garage sale
https://www.facebook.com/groups/tokyogaragesalepickuponly/
If you live in Tokyo, this is a very useful website for buying and selling furniture.
In Japanese law, if you wanted to throw a fridge and washing machine away, it would cost you a lot of money.
I hope you find these websites useful.