Half Price Books around 2000 definitely had multiple copies of Greyhawk, Dragonlance, and Oriental Adventures on their shelves around here. I ended up buying the ones I was missing then. I remember Ryan Dancey saying that TSR had a warehouse full of old AD&D 1e books in a warehouse as one example of them not thinking about their money (why are you paying to warehouse books you're not selling and have no intention of selling?) and I've assumed that Wizards sold off that warehouse full of stuff for pennies on the dollar just to get it off the books and that's where those books came from.I remember buying Greyhawk Adventures and Dragonlance Adventures for six bucks each at shop that also had the Wilderness and Dungeoneer's Survival Guides at that price. Unfortunately, I had already paid full price for both the survival guides. Worse still, a while later I'd return to that store and see them all marked down to $4 each!
(These charts now make me wonder how many of those books were actually returns on those titles. Yeesh.)