There is nothing any of us can do with the case number and you would need to provide that with the Apple contact that you used to get the number.
Here is the support article that you may have already used and the way to start a request to review the Activation Lock. That is the only option as you are unable to remove the lock on your own without the original owners Apple ID and Password. Apple does not get involved with third party sales that result in receiving devices with the Activation Lock, but in cases where the device is yours and you can provide the receipt with serial number and location of purchase, they may be able to help.
How to remove Activation Lock - Apple Support