The seller should have included a bootable OS X system DVD that would work in that computer; preferrably one that matches the OS X installed in the computer when you got it. Have you contacted the seller?
If you can find the serial number and any other model identifier of the iBook portable, you could probably get a Replacement OS X Tiger 10.4 system DVD from Apple for about $20; a white-label disc that should work in an old iBook G4. This should not be an original install-restore disc specific to the iBook, but there may be one available. (Originals shipped with a Mac were gray color and say the computer name, and those discs have a part number related to the Mac models build year, etc they shipped with.) A 'replacement' white disc should work with more than one specific series of Mac that could support the OS X.
As these computers get older and farther away from the date of manufacture, replacement discs or copies of original software kits, no longer made, are harder to come by. However the white label 'replacement' from AppleCare, may be the ticket, if you can order one and if they still have them. And, if it works.
With a system disc, you can erase and install (or secure erase, reformat, & install, then update) a system on the hard disk drive. By doing the secure erase, the Disk Utility will write a pass of zeros over the old content and that can help clear up old issues in a questionable used computer's hard disk drive, but you'd need an OS X install disc. You can call AppleCare or Apple Sales Support (not the main store or main online pages, its not a regular item) and if you are lucky, you may talk to someone who knows these exist. A few ASC discussion threads have mentioned these, and people who bought them posted pictures of the actual discs.
There are some ways to attempt to reset an admin password, usually with the install disc for OS X; but another way using Single-user mode or command-line Terminal, to go into unix and change settings at a level beneath the graphic user interface.
•TidBits: Five Ways to Reset a Lost Admin Password
https://tidbits.com/article/14437
+see comments post link, for Carlos single-user idea
https://tidbits.com/article/14437#comments_22009
not recommended for casual use without knowledge
•With an OS X disc, how to reset lost passwords:
http://support.apple.com/kb/ht1274
You'll have to search and reword for other online ideas.
If you bought the iBook from an auction, suggest they
should include the disc, since it implies a license to
have the system software installed. It is the proof.
The replacement OS X install disc for Tiger or Leopard
(depending on the computer, what it may use best) are
ideas, so ask AppleCare about them. If you can, get
both, so you may have one that does the job.
PS: If someone set an Open Firmware password, that
is another matter to access the locked computer. At a
level of security higher than Admin, near impossible.
Almost as bad as encrypting the hard drive.
Good luck & happy computing! 🙂
