Hello, dear iPhone help team. I bought an iPhone 6 before 3 months from Ebay. Yesterday I tried to upgrade my phone to iOS 9.3 beta and after the installation I got a message in iTunes: This iPhone is currently linked to an Apple ID (s••••@icloud.com) Sign in with the Apple ID that was used to set up this iPhone.

I do not have access to this iCloud ID. I used other Apple id to download apps and games from App store. I tried to enter it, but it was not accepted.
I also tried to activate my iPhone via Wi-Fi, but I got the same message: "This iPhone is currently linked to an Apple ID".
I also tried to contact the seller on Ebay to give me the iCloud id, but he didn't reply yet. Now I'm stuck on this screen, and I do not have access to my iPhone.
Is there any solution to fix this problem and recover my phone? My phone is bricked and I can't use it anymore?
Please help me.