How can I reset my iMac G4 to factory settings without the startup disk?
You need to contact the seller and see if they have the original OS install discs, as they'll have iLife and a correct version of OS X. Barring that, try to get ahold of a set of retail OS install discs for anywhere from 10.3 (Panther) to 10.5 (Leopard). I'd suggest you get the newest your system will support. (It's important to look for retail discs so you don't have to spend extra energy trying to match your Mac's exact specs and model with the one the discs were shipped with.)

How to choose which disc to get if you can find multiple OS versions?


10.2 - iMac G4s shipped with as low as OS X 10.2 (Jaguar), but an OS install/upgrade disc won't necessarily have the drivers to support your machine, and some iMac G4s may simply be too new for 10.2 to support them at all. 
10.3 - This is the lowest you should go if possible. Look for install DVDs (rare) or packs of install CDs. (I think there's two or three CDs to install the OS.)
10.4 - Make sure you get a retail version of 10.4. This is when Intel Macs were introduced, and an Intel Mac's OS install disc will simply do nothing in your PowerPC iMac. Look for DVDs. (Code-named Tiger.)
10.5 - Not all iMac G4s support 10.5 (Leopard). Anything with a processor slower than 867 MHz won't get Leopard. If you have an 867 MHz or faster CPU, you can use any retail Leopard DVD (these discs are for both Intel and PowerPC). As with Tiger, be wary of system-specific install discs, as most (if not all) Macs that shipped with Leopard were Intel Macs.