I have found Craigslist to be good for iMac G3 and G4, but anything else people seem to expect WAY to much for. In my area Power Mac G4 and G5 systems routinely are posted for well over $100, the same goes for iBook, but iMac G3's can be had for a few bucks.
Sadly in my Craigslist area the going price for a G3 iMac is $40-60. $40 w/o discs, $60 with.