Bought an locked iphone 6s plus on Ebay, What can I do?

I had bought the iphone 6s plus on eBay last week. It is still locked, I did contact the seller saying it was locked. The seller's stuff was still in the phone, like pictures, messages etc. I had tried factory reset hoping it'll erase everything... It did, but then... it said "Activation lock" above and asks for Apple ID. The seller had given me his info, like his apple ID and password, I tried removing it on Itunes. And I did. But It's still asking!! The seller does NOT know how to unlock it either, That is why he was selling the iphone. We both tried unlocking it, but it's just locked. I dont know what I did wrong, Help me?