Book Signing Etiquette

Is it gauche to ask an author to sign an old book, instead of purchasing a new one at the signing? I’m not sure where I come down on this, because it seems the writer should be thrilled to see a loyal reader. However, the purpose of a public appearance is also to sell new books, a tiring and tedious process, I’m certain. What do you think about this? Leave the old books at home? See the many responses to this post in Publisher’s Weekly. Shira S.