If it is in the great condition you describe, and if you can get a letter from Bushmaster saying it left their factory as an assembled rifle prior to the ban you could sell it for the following prices:
$1300-$1400; you'd be giving someone a pretty good deal
$1500-$1600; would be a pretty fair price
$1700-$1800; is high, high end and you wouldnt sell it right away, but there are "some" people who would probably pay that.
Oh, and so you know, you may have a harder time because the upper is not really that desirable to a lot of folks. Short barrel like that kill velocity and velocity is king for these bullets to maximize their ballistics.