Buyer beware !!!
I would be a bit carefull, if I were you. I like eBay, and use it a lot. I have only ever bought a laptop once, off eBay and unfortunately the description didn't mention the fact the screen was washed out, and needed replacing. (G3 Powerbook,Wallstreet - £112.00)
Whilst, it would be possible to use you Panther install disk's, you're software license is for only one Mac.
You might get one for around £150, but they generally go for more than that.
Have a search for "clamshell ibook", that might return more results.
Remember to check the specs however, you need at least MB of ram if you want to run Panther on it.
If you have an option of waiting and buying new, then that may be the better option.
By the time you have added all the unseen extras, extra RAM, Airport card if required, OS install disks etc. you might well end up spending nearly as much as you would on a new Mac.
If you do bid on a Laptop on eBay just make sure you read the ad very, very carefully, there are quite a few Laptop Auctions that seem genuine, but they are not selling a Laptop they just make you think that they are...they are actually selling you an address for a website where you are meant to be able to get one for free!!!