I've done many G4s (The Successor to your iMac G3). The CPU seems to be non-removable. I've tried everything I can think of. I'm sure the older iMac G3s and the eMac are similar.

I don't know who your buyer is, but I would assume to send them with the CPU on if possible.

If you use a large buyer, they'll probably be able to give you a value, or some advice.