Not trying to piss on anyones chips, but mostly it comes from pastoral herding breeds, border collie, rough/smooth collie, oes, beauverons etc etc some non kc beardie(but that more than likely came about from a outcross to border collie) great Danes and a few hounds also are merle. But pretty well it originates from a pastoral herder.
And my bet this dog is merle from border collie type, as it's most common.
Good luck with the sale, would be nice to see a picture.
Yis Sirius