I have a 1999 ATK 605 and really like it. I've put about 1000 miles on it with zero issues.
I got it for dual sport rides. I had tried a DR650 (too piggie), a plated XR650R (excellent, but the kick starting can be a pain), a KTM 525 (excellent, but too much maintenance when racking up big miles on dual sports).
I really like the low maintenance ... holds ~3 qts of oil and it goes thru the frame so it has a chance to cool off, air cooled (no radiators to mess up, no water pump seals), valve adjustments are far between.
Motoexotica (sp?) is semi-local, has parts, and will work on them if you need it. ATK (the factory) or the guy in LA (American Dirt Bike) also has plenty of parts.
But for that year model and that price it doesn't make sense. If you find a 1999-2002 model with less than 1500 miles, those might bring $2800-$3500. But as soon as you really use the bike, scuff up the engine cases, scratch up the plastics, get 5,000+ miles on them they drop to the $2000-$2500 range.
Did he put motard wheels on it? Would you have motard and stock wheels? That's worth some extra money if so.