Found one on Craig's List for $125 and a 2.5GHz model for $400. The difference from a private party and an established dealer is the warranty:
PowerMax:
"Our guarantee:
Each used Mac comes with a 90-day warranty, plus these additional benefits:
First, if it arrives and isn't working as it should, we'll send you a return shipping label and replace it at our expense (some conditions apply).
Second, if at any time during the first 90 days after the purchase of a pre-owned Mac you wish to send it back, we'll credit you the current sale price for the item IN FULL toward the purchase of anything else we sell! (Less a $25 restore fee. Limit one exchange per customer.)
We also offer extended warranty packages which will increase the warranty on your used Mac to either six months or one year!
For more information on our used Macs, check out our Used Mac FAQ"
It's insurance.
Disclaimer: No, I don't work for them. But I am a customer.
Dale