I miss my Aurora's Guide to the Realms. That was my all-time favourite D&D book purchase and some bastard player borrowed it and lost it. I was so pissed at him that I made him pay for it and only found out later (when trying to re-buy it) that it was a collector's item worth three times what I asked him for it, so I never ended up getting another one.
I wouldn't mind picking up a lot of the Greyhawk stuff if the PDF's were done really nicely. The problem with selling them as PDF's, though, is that they'd probably just be crappy scans like they used to have of some older stuff. In that case, I'd pass, as I can get crappy scans for free elsewhere.