I would not take a chance on buying that frame, no matter how the seller described it. Too many ham-fisted owners over-tightening CF for me to trust something like that as cosmetic, rather than structural, especially concerning the seat mast in particular.

That said, I'd inspect by sound -- rap the carbon lightly with the handle end of a screwdriver along the seat mast. If it sounds different, more a dead *thunk* than a sharp *knock*, near the crack, frame would be suspect...