You can probably get the best out of your phone by posting a local ad on http://craigslist.org. Always aim high and take the best offer. People are always gonna try and haggle.
sold my cracked 16gig 3G on ebay today for $250 buy it now, should have asked for more, the guy bought it within 10 minutes of me putting the auction up.
I wonder how much my 16gb 3g will go for after the new one comes out (I'm not selling it before hand, would miss it too mcuh and not sure when I can get the new one so may be longer than whenever it comes out).
It's almost 2 years old (We got it a month after the iphone came out), it's got a crack in the back (typical from connection port) but it works fine and the screen is pristine (though dirty but that can be cleaned off).
sold my cracked 16gig 3G on ebay today for $250 buy it now, should have asked for more, the guy bought it within 10 minutes of me putting the auction up.