Nope
"There is also the possibility that this is a law enforcement sting operation. The fact is that the OP knows or should know that you cannot acquire a new Nikon D90 for $600 off list price."
I HIGHLY doubt that.
Now I don't claim to be an expert (or an amateur, for that matter) in laws and law enforcement, but c'mon.
The "sting" situation sounds an awful lot like entrapment. Think of the situation: You see a killer deal on craigslist (Not THAT killer, same thing going on amazon used for just $75 more than this guys price) and figure you want to check it out. You buy it, and all of a sudden you are arrested because it should have been obvious it was stolen merchandise? PLEASE.
What if the seller was just stupid? Or REALLY needed cash? Or got it for a gift and did not know how much it was worth? The law cannot assume guilt.
OP: sounds totally legit to me. Bring a friend with just in case. Dont let him touch any cash until you fire off about 50 shots, try all the settings, try it with a few of your lenses, turn it on and off a few times, etc. Make a list of things to do in advance, before you go to meet him.
Good luck and have fun with your cheap new toy!
[edit] - I have bought 90% of my camera gear off craigslist for similarly discounted prices. Including a Canon 50D for a savings of about $300, a Canon 17-55 f/2.8 for about $350 off, and all of my flashes for dirt cheap. Everything has worked wonderfully and I have not had ANY problems. Then again, I live in Minnesota, land of the kind.....