Hello leegero,
Sounds like a chorus, but I'd try Craigslist.
Two things to note: 1/ Right now the used computer market is dead. It was great through May, and I expect it will pick up in August when the kids (and their parents) go back to school, and 2/ Sometimes you get a lot of bogus e-mail responses from jokers who have too much time on their hands. Not much you can do about it except be prepared for lots of dumb questions and people who promise and don't show. I think back-to-school will cut down on this, too.
I sold a couple of G4s this past Spring, and I've got a few more ready to go. Macs bring out some nice buyers; I really enjoyed the ones I met, and they seemed happy to test drive their Mac before they plunked down the cash. Haven't had a complaint.
Everybody wants a laptop today, too; that should help.
Good Luck!
Jon