If you have need of one, keep it as a server. I currently have a Sawtooth as my office server, will upgrade that with my G5 when I eventually switch to an Intel desktop of some sort. Power consumption will increase, of course, but not by too much if it's mostly hosting files and not running many processes...
Otherwise, yeah, Craigslist. I've bought and sold a lot of older hardware through there. I wouldn't sell it to a reseller, as you'll get maybe 1/3 of what it's worth.