Actually I did talk to the person but the seller told me that the person sold them the phone is not answering there email. They won't give back the money. It's there anything else I should do? Or maybe is there a chance that the phone can be unblocked or open line?
did you pay it by using your credit card? if so, contact your bank for a chargeback. (you can certainly go through this pathway if your card is visa or mastercard, not sure about others. i know there is a japanese one, but i'm not sure if they provide such service.)
this is a fraud, you can get police involved if you want to. in australia, when you sell a phone to a business, say 'cashconverter', you would need your ID and everything to prove your identity, so they can track you down, should anything wrong with the phone. not sure whether that's the case in JP, if so, it should be easy for police to track that person down and get your money back.
there is nothing legal you can do to unblock imei since the block is placed on carrier end, where they prevent you from connecting to their network. you can try to sell it on ebay to people in other countries since imei block is usually carrier/country specific, however the phone is from softbank, which means it's locked to softbank and won't be able to use in other regions of the world anyway. In conclusion, you might need to sell it on ebay as parts to get at least a bit of your money back. I would encourage you to state the status of the phone clearly on ebay if you chose to sell it there as parts.