unfortunately the price IS fair. I have not seen a 3GS in this good condition sell for $400...
If only this wasn't updated to 3.1.3 I'd consider buying it from you. It is fair to note that this phone CANNOT be jailbroken or unlocked, which you have to admit is one downside.
But this is your sale thread
would you trade straight up for an HD2? (if i happen to know someone on att that wants it)