ifixit.com will buy back used PB's just go here to ifixit.com
and fill out the info and they will email you with the offer.
its good for 30 days.
They offered me $1200 for a 1 1/2 year old perfect TiBook which is about 40% what i paid for it.
Powerbooks hold there value very well. As long as its all in working order. If its under a year old you can still buy AppleCare for it as well.
Post the Full specs and im sure there are many who will offer to buy it.