Upload & Sell: Off
I wouldn't be worried buying a gray market flash from a reputable dealer. B&H, for example, has an "Import" SB-900 for $469, versus $638.95 for a "USA" version.
$299, though, sounds too good to be true, and as the old saying goes, "If it seems to good to be true..."
I have visions of a high-pressure salesman in a smoke-filled office in New Jersey calling to confirm your order, and asking you if you want the $200 dedicated flash foot for your camera model. Oh, and the special $50 AA alkaline batteries that prevent blurry pictures by giving faster flash recycling.
Hopefully, though, all will be okay in your case and I'm just having paranoid delusions.