Great lens from what I've read. I wanted to get one for quite some time but haven't got around to it. Like mentioned, it won't AF on your D5000, but on a lens that wide AF isn't as crucial. It's probably pretty easy to focus manually.
I would just buy it new. For $50 more you get a warranty. If Tokina is anything like Nikon, their warranties don't transfer. If it was significantly cheaper then used would be the better option.
I don't understand why people try to sell used items for only slightly cheaper than what you can buy them for new. I see this all the time on Craigslist. General rule of thumb is once it's purchased off the store shelves it loses 10%, and that's for a product that hasn't been opened yet. Since it's been used, and (most likely) no longer has a warranty, it's worth even less.
I wouldn't give him more than $450 for it, even if it was flawless. I'd rather spend the extra and get a warranty and piece of mind.