There is quite a lot of parts interchangeability between 12-inch G3 and G4 iBooks, and also between 14-inch models of the same laptops, especially hard drives, keyboards and LCD panels. But that's a broad generalization that you need to be careful about when selling parts.
The most important thing for you to do before you part out your iBook G4 is to ascertain exactly which model it is. The serial number is vitally important in doing this, as is the label Apple puts under the keyboard. Google Klantenservice, which is a German site that lets you put in a serial number and get the basic information.
The reason for knowing exactly which model your parts came from is that Apple made a lot of running changes, particularly in the last few iBook G4 models, in things like frames, aluminum shields, Airport cards, bluetooth, etc. Parts that will work in your model won't work in an earlier or a later one.
Then there are things that you seldom hear about, such as the fact that Apple used several LCD manufacturers for the G4 iBooks. One manufacturer, Chi Mei, used an LVDS cable that looks like the one used by the other manufacturers. But it will not work with another manufacturer's LCD, even though it will fit, because the wiring pattern is different. This changed, thankfully, with MacBooks. But for iBook owners with broken Chi Mei LCDs, it's important to know that they either need another Chi Mei LCD or they need to buy a different manufacturer's LCD *and* the LVDS cable that works with it. This means an easy LCD swap suddenly becomes an almost complete tear-down to replace the LVDS cable.
So when you advertise your iBook's parts, be careful to tell prospective buyers exactly which model the parts came from. Knowledgeable buyers will thank you, and you'll save yourself time answering questions.