Time travel paradox. If you do something in the past that alters the future it results in that action being invalid. If you buy IBM stock early because of it's value it ceases to be valuable in the future. Also known as the you can't kill your grandfather in the past because if you do then you can't exist to kill him in the past blues.
You could by a few casks of certain scotch malts and keep them for private consumption though.
