Based only on what you've written here, I'd guess somewhere around $600-$700. It seems like (at least on MR) everyone's been trying to get rid of the 12" PowerBook since the MacBooks have been released. I was thinking about jumping on that boat, but I think I might keep my PowerBook for use when I just need to surf the web, write papers, listen to iTunes, etc. Then, I'll get an iMac as my primary computer. But who knows. I love my PowerBook, and as of now, I'm glad I haven't gone with the MacBook.
Anyway, back on topic. Since the MacBooks have come out and start at only $1100, it's going to be tough to sell a less powerful, used notebook for anything close to that. I've seen top of the line 12" PowerBooks with AppleCare remaining go for $900-$1000, so I wouldn't expect to get more than $700. If you do decide to sell, I'd try eBay, as (in my experience) prices there seem to be inflated a fair amount.