zen's advice is good. If it is a blown capacitor issue and you are handy with a solder, you stand a good chance of getting it up and running.
However, if it is the video card that is toast, you should pass on the purchase. Is it theoretically possible to replace the video card? Yes. Is it easy, or worth the effort. Probably not.
If you're able to inspect the machine, it would be good to bring a phillips screwdriver and open up the back of the computer to check the caps. If possible, you could also try to drive an external display. If the internal display panel is the problem, don't bother buying the machine either.
My $0.02, I'd stay away from this iMac. The iSight G5 iMacs are less problematic than the pre-iSight models, and you can find a good deal on many of these machines if you hunt around.
Were it me, I'd contact the seller and say, "Look, the machine isn't worth $120 in its current state. If you can't sell it at your asking price, call me back if you just want to unload it. I'll give you $50 for it." If they take you up on it, great. If not, move along, nothing to see here.
