I don't think I own very many rare RPG products. Sure, I have a bunch of Traveller stuff. Maybe some of that is rare? I think the rarest book I own is the Deities & Demigods book for AD&D containing the Cthulhu Mythos and Nehwon deities. I picked it up used for the fact it had deities in it. I had no idea there was a whole controversy surrounding the book.
And if you want to find out more about that controversy, I suggest you check out book one of the Designers and Dragons series. It's a great read!