When I saw the thread, I thought "Geesh, how would Amazon get these books?" I can't image HQ's having a direct link to Amazon so I guess the majority come from 2nd hand, etc., dealers.
I didn't realize Amazon was like that, I thought they only sold new, not "recycled" items. Sort of like Ebay without the auctioning. Betcha a lot of the items on Ebay came from Amazon or vice versa.