hehe... why not keep it as a machine to fall back on, should something go wrong with one of the other ones... also, an iBook certainly gives you more mobility. Can use it outside, in a café etc. ...?
I can relate... my wife and I recently bought a MacBook (which is sweet). I am keeping the iBook for my commute by train and also as a backup device - I use Chronosync for the address book and ical settings and sync O tunes for iTunes backups. So far so good.
craigslist, especially if you live in a large area (Denver definitely applies).
No ebay fees, no paypal fees, cash in your hand!
I first tried it about two months ago, and am literally blown away at how effective it is in Salt Lake. Especially good for super large and heavy stuff that you don't want to mail.