For me, there is
Yokohama Mothers Group, https://www.facebook.com/groups/364973500356290/
Japan Facebook group Garage Sale- all things kids https://www.facebook.com/groups/japangaragesale/
And craiglist tokyo: http://tokyo.craigslist.jp/search/baa
They are all good for scoping out bargains, or selling unwanted/unneeded things. The only downside to these is that they mainly consist of foreigners so the selling pool is limited. I always need to find someone with a child of the same sex, and a size or two bigger- so it can be a bit hard to find.
I finally decided to bite the bullet and try Yahoo Auctions Japan. I was a bit worried due to my limited Japanese- but with the help of google chrome translate it was actually a breeze. When I signed up It needed a credit card, or online bank account, but somehow I managed to by-pass it (I am not sure how). Maybe because I signed up for Japan Net bank- but didn't follow though?
Either way, the next time I logged on to yahoo auctions I was allowed to bid. I had read online that if you bid over 5000yen (including postage), then you have to pay yahoo a fee. I am not sure if it is true or not, but just to be safe I don't plan on going over the 5000yen mark.
The first item that I bid on, and won, was a gorgeous little formal outfit for Pi. I knew the likelihood of finding a formal outfit, for the right price and in the right size was slim on the foreigner groups, so I tried it on Yahoo. It ended up being a simple, in great condition and great value.
It is like Ebay in Australia/U.S/U.K- When the item is listed, it already has the postage price, which is SUPER handy (so you know the total of how much you want to pay) and you bid on the item (some you can 'buy it now). Once you have won the item, Yahoo sends you an email to let you know. You log on and get the bank details (don't click on the "i've made payment" button though). I printed out the page (it even works out the total to pay) and paid via my Japanese Post office Bank account. I will mention though that when I bid on the item I am sure to check what payment options are available:
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Payment, delivery
Successful bid price is paid in advance. Exhibitor after confirming the payment of the amount, it will ship the goods.
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I made payment, then the seller sent the item and after I received it I had to log in to confirm the receipt. Then all finished.
It was really easy, and the item was in good condition. It made me really happy that it was easier than I thought it would be.
There are so many items on there so if you can deal with it being in Japanese (or poor internet translation) I think it is a great place to pick up some extra things.