Keep:
PHB1 (x2 copies) (You are going to need these when WotC discontinues online support for 4E...which they will. Anyone who thinks otherwise is pretty foolish. Multiples always help smooth things up around the table)
PHB2 (One of the better player based supplements for 4E)
PHB3 (Not as good as the first two, but still real good)
MM3 (Top notch monster supplement.)
DMG1 (If you keep three books it's this one, PHB, and MV)
DMG2 (In a lot of ways this is even better than the first DMG.)
Dark Sun Campaign Setting (Just an all around great book even if you don't play the setting)
Dark Sun Creature Catalog (More monsters and fun to play against! Great book!)
Madness at Gardmore Abbey (One of the best adventures for 4E. That isn't saying much, but it's actually GOOD!! Keep it!)
3 Essentials Dungeon Tiles Sets (I'm a softie for dungeon tiles)
Heroes of the Fallen Lands
Heroes of the Forgotten Kingdoms
Rules Compendium
Monster Vault
Monster Vault: Nentir Vale
Sell/Give away:
MM1
MM2
Adventurer's Vaults 1 and 2
Heroes of Shadow
Heroes of the Feywild
Forgotten Realms Campaign Setting
Neverwinter Campaign Setting
Tomb of Horrors,
Dungeon Crawl
Classics 4E adventures
DM's Kit for Essentials.