The seller (in palo alto) said he had to drive 200 miles to pick it up which suggested that it came from here:
http://www.holsworthwoodworks.com/apps/photos/
Since no one else within 200 miles sells slabs of walnut like this...
But I have no way of confirming the original source for sure, but if my guess is correct, then the walnut would be Claro from California.
I was looking through the gallery to see if I could find these slabs pictured, but it's hard to tell. Likely it would have been a piece like this:
http://www.holsworthwoodworks.com/apps/ ... =168816972
and then cut down the center to fit in a vehicle.. and then some small bits of it may have been used for a small project before the craigslist seller said he was tired of his son's wood taking up space in his garage.