How/where to sell iphone 4s?

I have a Softbank iphone 4s 16gb, the 2 year contract is up in May 2014 (I think) but I'm leaving Japan at the end of January.

As Softbank won't unlock their phones, I'd like to sell it before I go. It'll cost me 10k or so to break the contract. The phone is already paid for.

I've seen ads on CL, but ideally I'd like to just go to a store, sell it and get cash straight away. I thought there might be stores in Akihabara that do something like that?

Does anyone know of any stores that buy second hand phones?

You can sell at Sofmap Akihabara kaitori center. Do you know where the shop is? It's near Torano-ana B Akihabara on Chuo-dori Avenue. Ask any of sofmap staff if you have lost the way. Only Kaitori center of sofmap buys iPhone4s(no any other shops of sofmap buy). I think iPhone4s is the max price of 16,000 YEN as shops buy. Clean iPhone before you sell & bring all adapters USB cable, manuals and box with your ID(Gaijin card or Zairyu card) together.

Thanks very much for the information. I'm sorry it took me so long to reply, I didn't realize it had been moved into this part of the forum!