I've told him that if he's unhappy he should open an item dispute with ebay and return it to me for a refund. We'll see what happens.
update Friday - heard nothing more about this, which leaves me sure it was a quick attempt to scam for a partial refund. Meanwhile the buyer of the iMac G5 has left positive feedback, so at least that one's OK. Had a look at upping the hard disk size on the G4 but it's fairly complicated and I doubt it's worth the hassle. I'll try to get a suitable box today, and re-list it once I'm sure I'll be able to pack it if it sells.