The following data is taken from Amazon.com on Oct 28, 2008. It probably doesn't mean that much. I was having fun collecting it. 
#1,085: Adventurer's Vault
#1,449: D&D 4E Gift Set
#2,734: 4E Player's Handbook
#2,863: Forgotten Realms Player's Guide
#4,949: 4E DM's Screen
#5,507: 3.5E Player's Handbook
#6,954: Forgotten Realms Campaign Guide
#9,337: 3.5E Dungeon Master's Guide
#12,849: 4E Dungeon Master's Guide
#15,558: H2 Thunderspire Labyrinth
#17,234: H3 Pyramid of Shadows
#19,494: 4E Monster Manual
#23,004: Scepter Tower of Spellgard
#24,656: 3.5E Monster Manual
#25,771: H1 Keep on the Shadowfell
#30,914: 4E D&D Starter Set
#445,483: The Sunless Citadel (3e adventure)
#448,647: Pathfinder #1: Burnt Offerings
#561,780: Pathfinder #14: Children of the Void
#747,380: Pathfinder #13: Shadow in the Sky
#948,303: DL1 Dragons of Despair (1e adventure from 1984)
#1,025,107: WG7 Castle Greyhawk (terrible 1e adventure from 1988)
#1,652,435: Pathfinder #15: The Armageddon Echo
Cheers!

#1,085: Adventurer's Vault
#1,449: D&D 4E Gift Set
#2,734: 4E Player's Handbook
#2,863: Forgotten Realms Player's Guide
#4,949: 4E DM's Screen
#5,507: 3.5E Player's Handbook
#6,954: Forgotten Realms Campaign Guide
#9,337: 3.5E Dungeon Master's Guide
#12,849: 4E Dungeon Master's Guide
#15,558: H2 Thunderspire Labyrinth
#17,234: H3 Pyramid of Shadows
#19,494: 4E Monster Manual
#23,004: Scepter Tower of Spellgard
#24,656: 3.5E Monster Manual
#25,771: H1 Keep on the Shadowfell
#30,914: 4E D&D Starter Set
#445,483: The Sunless Citadel (3e adventure)
#448,647: Pathfinder #1: Burnt Offerings
#561,780: Pathfinder #14: Children of the Void
#747,380: Pathfinder #13: Shadow in the Sky
#948,303: DL1 Dragons of Despair (1e adventure from 1984)
#1,025,107: WG7 Castle Greyhawk (terrible 1e adventure from 1988)
#1,652,435: Pathfinder #15: The Armageddon Echo
Cheers!
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