You may have to get & use a quality DVD player cleaning disc
to see if the lenses in the iBook can be the source of this issue.
Did the computer include the operating system install DVD?
That should also be an important part of the purchase, also
the 'original install-restore' DVD media discs, w/ gray label.
There are a few things you could try, in addition to cleaning the
optical drive player's lenses, that may help. An external DVD
drive may or may not work. USB2.0 or slower could balk. And
if you need to boot from a System DVD, that won't work; may
need a FireWire based DVD combo or superdrive (hard to find.)
Some have suggested to reset the PRAM (NVRAM) using all four keys on
startup to force this to occur; & let the computer start sound (bong) 3 times.
"...to reset or 'zap' the PRAM - hold down the Opt + Apple + P + R keys immediately on
startup - Continue hold the keys till you have heard three bongs. Test the drive again."
•My Optical Drive ejects discs after I insert them - iBook G4 12" 1.33GHz - ifixit.com:
http://www.ifixit.com/Answers/View/6415/My+Optical+Drive+ejects+discs+after+I+in sert+them.
However, some suggest that you reset PMU (power management unit)
•Resetting PowerBook and iBook Power Management Unit (PMU)
http://support.apple.com/kb/HT1431
Hopefully this will help. Various hardware issues are a reason why auctions are not
an ideal place to buy a previously owned Mac. Companies who repair or sell them
directly to a customer, online, offer greater value, set price, & a limited guaranty.
Good luck & happy computing! 🙂