That is about the same price as refurbished S1's were going for about 6 months ago.
So, it could be a legitimate deal but make sure the seller knows that you are looking for an S2. The part numbers are easy to confuse and often are.
Good luck to you, that is a very reasonable deal.
-Mr. Wigggles
Ps. put it through the ringer in those first 90 days looking for problems and don't mess with it after that.
Pps. You seem concerned which leads me to believe this is some sort of eBay/internet deal. As always, excercise caution.