It's mobile 1st. Take a picture of the item you want to sell, add a price and it's on sale. They've done nearly everything possible to make things as seamless as possible. When a sale happens you get notified where and how to ship it. They partnered with the post office and the 50k+ convenience stores so you can ship from the stores. At both locations they auto print labels via QR codes and offer fixed box sizes. I think they do a little bit of escrow (not 100% sure the actual mechanics) but I believe the buyer pays immediately but the funds are not forwarded until the buyer approves the item. (really not sure about this part)
Certain things seem like it might only fit Japan. The convenience store partnership is huge since, at least in populous areas there is always multiple within 2-5 min walk. Even post offices are relatively many. I also don't know to what extent a more trustworthy populus helps.
They are hiring like crazy and hiring foreigners as well if you want a job in Japan. They're probably the #1 company for running dev meetups in Tokyo and those meetups feel like SV, all English, in rooms that might as well be at Google Mountain View including food and kitchens etc..
AFAIK they've had no luck replicating their success outside Japan