The other things have been addressed already, but in regards to the 10.5 - I would ask whether he has loaded it on, or whether it actually includes the media. I've seen many people say xyz, but the reality is, its loaded on the machine with no licence - which is illegal since they're not transferring the licence to you at point of sale.
Regarding the price, personally, save yourself some heart ache and purchase a brand new MacBook, for around the same price you can get a 2.2Ghz jobby including iLife and Leopard, and knowing its all kosher and working rather than risking picking up a second hand one and all the risk it entails.
For me, I find unless the amount is truly low in cost, its not worth the risk.
Oh, and btw, the processor on the computer advertised is ancient. Technology circa 2 years ago. Anything less than a Core 2 is a waste of time and money.
edit: don't be stupid, avoid PPC stuff like the plague, its a dead end.