Given that Ray Bradbury spent so many hours signing books, some people say that an unsigned Bradbury is worth more than a signed one. I've never been into collecting signed items, but I do have a few things signed by Ray. Here's a couple of them:
The typical Bradbury declaration "Onward!" on a paperback copy of The Cat's Pajamas. And a simple "Love!" on a postcard from one of his stage plays. It was partly because of this card that I knew the name Alan Neal Hubbs. When I met Alan a few years later, he was astonished that anyone from the UK had ever heard of him.
Somewhere, I believe I have another signed item, where Ray wrote one of his other declarations, the odd "Mad love!" - which I have always assumed was inspired by the Peter Lorre film.