A similar thing happened to my Gateway Solo notebook a few years ago - happened after a sharp hit, but can also occur from vibration, bumps and knocks, and of course - a lot of opening and closing! Basically it sounds like the inverter card is going (or has gone), or has become disconnected slightly.
Whilst it's repairable, generally manufacturers like to replace an entire screen. I've still got that laptop, and it will work at certain angles, or if you put a bulldog clip on one side of the screen - but it's a thin plastic casing, as opposed to the PowerBook which is metal - so you could be okay.
As with any faults with computers, expect them to get worse over time - it is a general wear and tear problem...but if it's a good deal...go for it!