You'd have to try and see what recourse you may have through
the eBay rules, since the computer should've had its Mac OS X
install-restore disc set, as part of the license to use the System.
A later retail DVD full install disc such as Leopard 10.5 may do
if you were to see about finding one from an Amazon reseller
or maybe someone who has the correct (retail) either Tiger or
Leopard. The system depends on what the iBook G4 has for
hardware; if the iBook G4 is an early one with smaller than an
867MHz PPC processor, it won't upgrade past Tiger 10.4.11.
Apple had been a spotty source for a generic white label DVD
replacement disc to install in supported PPC Macs, a full version
of Tiger 10.4, on one disc; and another disc, for Leopard 10.5.
The source of these had been AppleCare or Support, not retail.
And some persons at Apple were unable to verify their existence
until actually entering purchase codes into their database, as if
they were invisible, according to feedback from US vintage users
of PowerPC G4/G5 computers that did not have original discs.
So if you can locate a product Serial Number, sometimes in the
battery compartment, and call Apple Support or Applecare to
order a replacement OS X system installer on DVD, you'd need
to supply that serial number in an attempt to find out if they have
anything at all. And, also have a method of payment (a CC.)
Not sure how things differ on the England side of the Atlantic...
Here, on east side of the Pacific, things can be very different, too.
Usually grey-label discs for a specific series build model year
Mac won't do anything if attempts are made to use them in
another different Mac model. And would be an as-shipped OS.
Can't say as I have a workable resolution for your circumstance.
The PowerPC hardware limits you to no later than Leopard 10.5.8
and that should be only if the processor is faster than 867MHz. If
not, then the latest OS X is Tiger 10.4 from DVD, & these systems
get their last update via Software Update online, in a Combo file.
Perhaps you can better identify the computer, and see if a regional
Apple affiliate or specialist has access to a supported OS X installer.
That is the way to perform a complete erase, install, in most models.
Be sure if you buy a system DVD that it is a retail one and not a
disc set from a different or other Mac computer, as they rarely work.
Good luck !