Where do you sell your cell phone in HK? cell, phone, Where, sell

Mini

I bought my cell phone from HK and now I want to sell it and get another one as I will go to HK soon.  Where do you sell yours usually?  And how much do you expect to sell it for?  Is there any website that I can get more info about this?

Mine is a SEW800i with an extra OEM battery + charger.

Thanks in advance.