I wouldn't recomend ebay. Last year I sold my G3 clamshell ibook for ~ $ 800. If this isn't telling anything then...

Honestly, if used macs are sold for almost the price of new ones, on ebay, I rather save some money and buy directly from the Apple store or an apple reseller.
Personally I don't know why people are so crazy to spend so much money on old machines. Yes, Apple products have a great resell value and offer more quality/ reliability but these high prices (for used products) are just plain odd.
The only thing I use ebay for os buying lots of Apple "gadgets"/accessories which are often sold for good prices.
For examble, I got a black apple pro mouse for ~ $ 20,
a graphite airport station for $ 60,
a black pro keyboard (defective) for $ 5, which I was able to fix myself.
Hm, I don't know if these said prices applies for the US, whatever ebay, but that is the impression I got from German ebay site, and I've been searching for a well priced, used G4 tower for months (on the German ebay)