Looks like someone dropped it and the rear piece shattered. You can replace it easily for about $45 from Amazon.
For $300, I would have the seller show you the OS and what not to make sure it's not an Android, and of course check the IMEI to make sure it's not bricked.
For $300, I would have the seller show you the OS and what not to make sure it's not an Android, and of course check the IMEI to make sure it's not bricked.