Ivory keys discolor with age. Sometimes they split. My piano (ca. 1930) had all of the ivory removed from the keyboard when it was restored in the 1960s and replaced with plastic. I don't think that they've used ivory for piano keys since the 1940s.
I do have a baby grand piano with ivory keys that was recently restored. The restorer recommended keeping the ivory keys and he carefully filed the sharp edges and polished them up real purdy. I would never have imagined that it would be illegal to sell.