The very simple way to ends this is to send him the cue as he requests. Do not sign or mark the cue in any way and have him sign an agreement that he is buying the cue as is as defective. At that point learn a lesson for the future.
Hearing not that it was stabilized wood we know where the rattle came from. The polymer that was used to stabilize did not accept the glue. I have seen some stabilized woods that you could not glue at all, it was like they made of Teflon.
No shit, I did not know that.
Great piece of info thank you. I have never heard a rattle like this before so this may very well be why.