iPad Removing slip case marks from back of iPad 1

Purchased an iPad4 on the Black Friday deal so I am going to sell my iPad1. I removed the Apple slip case and discovered the back has two markings from the case. One goes the length of the aluminum back where the closure flap lay. The other is an oval outline of the stand clip. These aren't scratches but the aluminum back is slightly darkened.

Anybody have any idea how to remove this "deposit"? Not willing to take any chance with an otherwise flawless iPad. TIA

Decided to try using a microfiber cloth and elbow grease. It cleaned up real nicely. Must have been real fine pieces of the case caught in the pores of the aluminum back. I had tried rubbing it off unsuccessfully in the past. Maybe the trick was using microfiber to adhere to microfiber.