Getting back to helping people with a second-hand locked ios device.. Google is directing thousands of people to this thread.
I was in a very similar situation after buying a 4S off eBay. It was linked to someone else's iCloud account and Find my iPhone was on. The seller didn't mention this in the discription, and wouldn't respond to messages. I was very upset to have paid $150 for a brick.
But I got it unlocked. The only way to unlock the phone is through the previous owner. Don't try the DFU route. If you already tried to restore the phone, or it came to you wiped you may simply have to wait for the jailbreak unless you find a way to get their email address.
If (like me) you got a phone with Find my iPhone active, but you can access the settings, then you have a better chance. Find the previous owner's email address (Settings - iCloud) and send them an eloquent, respectful email. You probably only have one chance to get it right. Include your full name, ask them if it was stolen from them, and if not ask them if they would mind taking a minute to erase the phone and remove it from their iCloud account. Explain that they don't need to give you their password, and include the instructions (below). I realize this won't work for some, but there isn't a software solution to this problem (yet). So focus on the people solution.
Three days after I emailed the previous owner my new phone was remotely wiped and unlocked.
http://support.apple.com/kb/PH2702