Apple switched over to including 2 restore DVDs with computers. These DVDs had Panther and all the preloaded apps on them. If you bought a Mac in 2004 and after MWSF, then the computer should have came with all the iLife '04 apps. This included Garageband. With the Restore DVDs that came with my PB last year all of the iLife apps except GarageBand and iDVD are on the first disk. Garageband and iDVD were pretty much the only apps on the second disk.
Have the buyer insert the first DVD and install Software Restore. From the Utilities folder under Applications, he should be able to reinstall all the software that came with the eMac. If that fails, he might have to install Garageband manually. This requires enabling hidden files for Finder and searching the second DVD. I have done this before when I didn't want to install all the programs I didn't use.