You will have to identify the model more precisely for constructive comment because there were so many PowerBook G4 models, including the first PowerBook G4 (titanium) that are essentially obsolete, several can run an OSX as new as Tiger 10.4.11 due to processor and other hardware constraints.
Buying one from ebay is kind of a gamble on a good day.
Dark spots on the screen? Rub marks or dead pixels?
Parts? powerbookmedic, wegenermedia, ifixit, OWC, & others.
Some of these also list repaired computers in their inventory.
You could view comparative information http://everymac.com on
all the different PowerBook G4 models and they also have a best
guess at the highest retail value based on function and condition.
A good source of offline information is the download database
from http://mactracker.ca (w/ versions for older systems, too)
The most RAM any of these could use, and basic specs, can
be found at everymac and also mactracker.ca
I was given what appears to be an aluminum PowerBook G4 12"
that needs significant work; the DC-in board, a hard disk drive,
perhaps a logic board solder; it also has a few dents. After a time
I was able to get it charged by finding where the power adapter
seemed to actually charge, then I taped the wire in that position.
But later on startup, I found the display worked but that was it.
Things made noise inside. I don't remember if I tried FW TDM
to attempt to see about the hard disk drive, from another Mac.
So anyway, you can get a repaired PowerBook G4 with a limited
guarantee of a sort from companies who repair Apple portables.
And they most assuredly are a better-bet on function than ebay.
At least they'd start out on their second life on four tiny feet.
Good luck & happy computing!