Apple sells the same iPad 2 + 3G for $419 refurbished. Those are typically like new, other than coming in a white box. I'd say that realistically, you should look for around $350 or so, provided your iPad 2 is in great condition. If you are going to list on Craigslist, I'd start at $400. This should give you sufficient wiggle room on the price. People like to think they got a deal. You also might get lucky. Again, considering Apple values it at $419, you might find someone willing to do $400. Remember, they add tax, so it's not $420 shipped.
Also, I'd make the move within the next month, before the next refresh. Prices will likely dip again.