600 $ is totally unrealistic in my opinion! I've just sold a 128GB (wi-fi only) space grey iPad Air with SmartCover for 500 $ Canadian. I was asking 550 $ and I had to lower the price to 500 $ in order to sell it. I sold it within five days though. I know iPad Air 2 launch is a few weeks ahead so timing was critical to me (and didn't want to take that risk).
I've noticed that second hand market for iPad has been flooded lately. So in your case, I believe you should settle for 500 $.
I'd list for 500 and count on maybe getting 400-450 from the right buyer. I tried to sell a 16GB ATT Air 6 months ago and didn't get the first crack for anything over 350. Most second hand buyers want cheap and they do not care about cellular data. They most likely won't even activate it on their wireless account due to having to pay a monthly service charge. Those who actually need a cellular capable iPad generally buy them new from the carrier. Not saying you won't find the right buyer that will pay what it is worth, just saying the chances are slim. Good luck though.