Just recently I purchased a second hand G4 tower and a G3 iMac. For a while I've been interested in Mac OS X and given I have a background in both Linux and Unix, it seems like the next thing to try out. So to that end I went and found some cheap Macs to have a play on. The G4 tower cost me $34 and the iMac $9 (w00t!).
The iMac came with Panther installed on it, the G4 with OS 9. In vain I have tried to get OS X onto the G4 with no success. It just won't boot off the 10.4 CD, even though it can read it. I might have to find a more legitimate copy ( something I was intending to do anyway ). I prefer to try before I buy, which is why I haven't bought OS X yet.
The iMac on the otherhand, had an existing installation on it. That's right. Someone's personal data, possibly their work information was still on this machine. And it was password protected which meant - no access to my own computer. Bummer. A bit of Googling later and I had things to try out. None worked. So I renamed the Netinfo database folder to old and thought I'd just see what happens.
OS X decided it was a brand new install and gave me all the normal set up options! Yay! I nuked the old account, all the data it had and created my own account. I threw in some extra RAM and away I went. My little iMac is great! Now I just have to sort the G4 out.
The moral is, if you buy a used Mac with OS X on it, and you can't get into it - try changing the netinfo database folder name so it has to create a new one - you might be pleasantly surprised.