Did you get the set of disks that shipped with this iBook? (not much of a trade if you didn't get those...)
You can try booting into single-user mode - hold the Apple-S keys, and you should see a black screen with test, scrolling to a command prompt. Type '/sbin/fsck -fy' without the quotes, press enter, and see if the result repairs anything, and says *** file system was modified ***, or something like that. Run the same command again, until no problems are found.
then type /sbin/mount -uw /
press enter, then type reboot and press enter again,
your system should reboot, and might be successful this time.
If the fsck keeps finding the same problem, and won't repair it, then you may have hard drive problems.
This is where your set of disks will come in. Put the disk 1 in your drive, and reboot - holding the letter 'C'. At the first screen, go to the Utilities menu, and choose Disk Utility. Click on your hard drive when the utility opens, and click on the Repair Disk button at the bottom of the screen.
Come back here with what you discover.
Serendipity is a lucky guess !