Also, paying $$$ for ultra rare game on eBay is just plain stupid. It used to be about the hunt. Finding something in the wild is so much more rewarding.
No, a holy grail is an item that is highly unlikely that you'll encounter, preferrably even rumoured to be vaporware. Otherwise it is just a strong desire.
I don't think I really have any holy grails on my wants list at this point. There's nothing I want that's so rare I'm unlikely to ever find a copy. The closest is probably Dino Strike for the Wii, but even those show up on eBay (usually absurdly overpriced) every couple months.