Classic Scifi Authors

I'm trying to teach my coworkers at a used bookstore which scifi/fantasy (mostly scifi) books we need to accept for trade. Right now they're using the standard age/condition rules; no more than about ten years old, no spine breakage, no cover damage.  That results in missing a lot of old classics and passing on current fabulous books because of condition issues.

So!  Here is my tentative list, massively shortened to be relatively easy to remember for those who don't read scifi much.  Feel free to chime in if I'm missing anything major.  I tried to keep my personal bias to a minimum here... but who can really do that, with scifi?  I also tried to keep it at 5 per category, but obviously that didn't work out so well.

Old stuff:
anything published before 1930 that isn't falling apart

• Robert E. Howard (Conan but also other stuff)
• HP Lovecraft (may also go in Horror or Classics)
• HG Wells (may also go in Classics)
• Jules Vern (may also go in Classics)
• Edgar Rice Burroughs (Esp. Tarzan, Barsoom/John Carter, but also other stuff)

"Golden Age": (1930s-1960)
most anything before 1960 if it's in pretty good shape

• Robert Heinlein (lots of YAish books, Starship Troopers, Stranger in a Strange Land, etc.)
• Isaac Asimov (lots of short stories like "I, Robot", the Foundation series - but only the ones that are by him, there are a lot of other authors that expanded his short stories into novels and mostly they're terrible)
• Ray Bradbury (may also go in classics) (Dandelion Wine, Fahrenheit 451, etc.)
• Phillip K. Dick (Do Androids Dream of Electric Sheep?/aka Bladrunner, etc)
• Frank Herbert (Dune series)

Modern: (newer than 1960)
use normal age/condition standards except for these authors

• Ursula K. Leguin (The Left Hand of Darkness, etc)
• Harlan Ellison (lots of short stories like "I have No Mouth and I Must Scream", etc.)
• Andre Norton (Witchworld series, etc.)
• Roger Zelazny  (Amber series, etc.)
• Orson Scott Card (Ender series, Alvin Maker series, etc.)
• Marion Zimmer Bradley (Darkover series)
• William Gibson (Neuromancer, The Difference Engine, etc.)
  

Current hot authors:
take anything that doesn't actually have pages missing

• Neil Gaiman (The Graveyard Book (YA), American Gods, Good Omens, etc.)
• Cory Doctorow (Little Brother (YA), Makers, etc.)
• Terry Pratchett (Discworld, obviously)
• Neal Stephenson (Snow Crash, Anathem, etc.)

For more resources:
Amazon.com's bestsellers in the scifi/fantasy category
The Hugo Award nominee List
Nebula Awards nominee list and Suggested Reading list
The Nebula Grand Master list