First time you sold your DnD books, for any reason.
Donated my 3x stuff to the public library when I was moving out of MN, partially because 4th had just dropped but mostly because the SRD made the books obsolete. Not sure that I got rid of the Deities book, because I love the art, but I think I parted with it in the hopes that some kiddo gets a chance to do as I did.
Sell your DnD books. Please.