That is probably a decent deal (though it's certainly not a hybrid!).

I understand that mountain bikes in particular depreciate in value pretty quickly, as there's always that "next best thing" on the current model bike, and ownership tends to be short-lived, at least with a certain set of folks. With a bike this new, there's probably not a whole lot that could be wrong with it mechanically. I would check to make sure the wheels spin true, the brakes and shifters work, etc. Otherwise, it's more or less just finding something that appeals to you and that fits. The larger your Craigslist market, the more choice you generally have. Also, the more often you check Craigslist with some pre-set filters and searches, the more likely it is that you'll find a good deal before someone else.

If you want that Specialized mountain bike, I'm sure it's a fair deal. It looks to be just over half of MSRP, which is probably fair. It definitely doesn't have hybrid geometry, though, and the guys in the mountain bike forum here might be better able to give you an assessment of what the value of that bike is.