Maybe I have too much of a vested interest to be objective; but I think Mark could do very well out of those plates.
If there is an active heritage plate scene in SA as has been suggested, the disparity in prices between SA & Vic/NSW could narrow. If that was Vic or NSW 18 the rights to those would he at least $750k, perhaps more because of the "1" and the "8" numerals. Number 17, although a lower number, would be worth less.
Well played Mark 👍