I do not believe there is any question but that the arm in question began life as a carbine, hence there is little reason to suspect it is "cut-down", unless it was assembled from parts by Bubba. Is the gas-escape hole in the bolt in line with the hinge pin or ahead of it? Is the lower rear cut on the extractor (where it pivots in the sleeve) sharp-cornered or rounded? I'm assuming the sight goes to "20" and is marked "C"? Is there a little dip in the top edge of the slide at the sighting notch? Does the front sight look like B/R's pics?

That price indicates one of two things: extreme ignorance on the part of the seller, or an obviously buggered piece of junk. A faker, or a knowledgeable seller, would ask MUCH more.

Hope that helps.