Are Books Signed by the Author Valuable?

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I wrote recently about a record album I have that was signed by the artist and given to my uncle. But I wonder if being signed really means anything? Many years ago, I went to a poetry reading by a famous Russian poet named Andrei Vosnazensky. He read in Russian & then a local professor read the poems translated to English. I bought the poet's paperback book, watched him sign it & have had it ever since. But, I was talking to a local bookstore owner and he pointed out that there was really no way to verify the poet's signature. Maybe I could look up something that shows his writing, but for all a buyer knows, I signed the book. So, maybe things signed by artists & authors don't have any added value. Have you ever bought a book or record signed by the artist?