My friend is on a hunt for a used iBook/Powerbook but doesn't have much faith in ebay because of scams and so forth.
She is looking for a 600MHz + iBook (pref a G4 though) with Airport for under $700 US.
Second hand computer stores are a total rip-off changing $150 less then a NEW iBook and no warranty.
Graigs list is so so but they sell out 1 hour after they are posted.
Any other recommendations?