In reply to arie_subiyanto
This is the way it is supposed to work. This is a security feature that prevents restoring or reactivating a phone without the original owners consent. Stolen phones cannot be used again unless the original owner approves. People buying used phones must check the phone to make sure it will be usable. People buying from shady sources will eventually learn this new way and stop buying from those kinds of sources.
I have a iphone 4s which i bought from other.
The previous owner has upgraded the phone to ios7 when i bought it, which, later, I downgraded to ios6.
In other words, you jailbroke the phone. In addition to your activation lock problem that guarantees that Apple will not help you. There is no legitimate way to dowgrade iOS.
i face the same problem, although my wife is the first owner, but she can't remember what was the apple ID been used when she seted up her 4S (one and half year before she bought it). now i'm in the same situation, my wife can't remember the Apple ID, and i'm still looking throught the internet to find a solution for that.
i don't know how to use jailbreak or those softwares, and i prefer leagally way to solve it.
please advice ....
Read through this article to find if your device has an activation lock.
If this is what you have - the news is bad: