Helpful resources for online shopping and disposing-of / selling-off old stuff.
Amazon Japan
I would assume most people have heard of Amazon by now. :-)
Craigslist
I find Craigslist very convenient for disposing of furniture and large things that are difficult to sell and ship with auction sites.
MOO
Business Cards & Printing
Business cards are critical in Japan, ne! MOO will help ya out. More info here: Market, Market, Market - How to Be a Freelance English Teacher in Japan
Oikura
Sell Off Big Stuff
I usually sell my used stuff using Japanese auction sites like Yahoo Auctions, but I use Oikura for those times that I want to get rid of something large and cumbersome to ship (like furniture or a refrigerator). Oikura is a site that allows you to offer your used stuff to many recycle shops in one shot, which greatly helps to mitigate that "I'm getting ripped off" feeling when dealing with Japanese recycle shops.
The shops submit offers, and you can choose the best one to do business with.
Shipito
Shop online in the USA and have packages forwarded to you abroad
Shipito is a service that gives you a USA forwarding address. Shop online at USA websites (e.g. Amazon.com), and Shipito will accept delivery of your order in the USA, then ship it to your Japan address. They have come in very handy on a number of occasions!
I blogged about them here: How to Get USA Stuff Shipped to You in Japan - Living Abroad