Broken Powerbook: How to tell what's wrong?

I'm looking to buy a "broken" Powerbook from someone for pretty cheap. The seller stated that he was told by a Mac Store that all it needs is a new hard drive. When asked about what happens when it turns on, he said: "Acts like it's booting into OSX and then a black dos like screen pops up with a login prompt."

Are there any things I can have him do to figure out if that's the only problem?

i would strongly recommend against buying a broken powerbook unless it is *absolutely* difinitive wat is wrong with it, or it is a really good deal, and worth fixing no matter what. Or you could go to visit the seller with an OS X disk and do the diagnostic. thanks.