I think we might need more info, but with what little I know about a D-back Apex, my out of the box sense is, that's a little high. But you live near the city. Bike prices in NYC are high. Around where I live, a former high end, rigid fork MTB would sell for around $100 if it's in good condition, ready to ride. In Long Island, I'd expect the prices to be higher. If the bike is nice, I'd offer a little less and see where the owner goes with the offer.
Remember the most important factor, the bike MUST be your frame size. If not, I'd pass. If at all possible, ride the bike. If you can't ride it because of the bikes condition, or if the chain is rusty or if it's covered in dust and dirt, that price is high. Take into account any repairs that might be needed.