Apple had early '08 Macbooks (same as I have) on the refurb store the other day for $699. Yeah, these have the crappy Intel X3100 graphics, but it would be a FAR better option than any eBay/Criagslist purchase IMHO (and I buy lots of stuff on eBay, including my iMac I'm using now), as you would get a good-as-new Macbook and a year warranty.
Now the only problem is waiting for them to reappear...
The cheapest right now is $849 for the current Macbook. Just keep watching the refurb store!