That would based, firstly and largely, on the condition (of how worn it is), whether it's 1.83 or 2.0 (CD or C2D), white/black and the going rate of a refurbished of that very model from Apple (which most people, given two non-new computers with same specs, would prefer). Then factor in any upgrades you made, e.g. RAM, HD or Invisible Shield etc.
3 months-old "Like New" models of the 2.0GHZ Core Duo MacBook on eBay are hovering close to (and at times are just the same as) the prices of a newer model from Apple. I don't really see any consistent pricing strategy/buying pattern in the used MacBook market (on eBay, that is).
I'd say Apple rivals Honda in holding resale value. Furthermore, I think some dedicated Apple fans will pay almost the same price for a used MacBook in good condition as they would a new one. Dunno why. But YMMV.
P.S.: When all else fails, rely on the original packaging to boost the deal.