Waist: 38" Measured across (at center front button) then doubled
Length: 29.5" Bottom of collar to bottom of the jacket
Shoulder: 17" Measured across the shoulder, point to point.
Definitely more of a 36S than a 38S. The jacket waist measurement seems large compared to the chest, though, so I would definitely check with the seller that he knows what he's doing when measuring. Also, I just looked up the actual eBay listing, and there is no trouser waist measurement, so check it's not a mismatched suit. IIRC, though it's a long while since I looked, Brioni typically has a drop of 6, so the trousers should have a 32" waist if this is a 38 and a 30" waist if it's a mislabelled 36S.
The label & suit both look legit though, despite the measurements being a bit off. The seller's photography/display/sales blurb could all use some finessing, but whatever...