Could this also be translated as "The sales clerk has a white dress"?
Yes. And it can also mean that he is wearing it/them at the moment and we are looking at him.
Az eladó fehér ruhában van.
Az eladón fehér ruha van.
So the original and those location cases mean all the same?
The only difference -nak and -ja also means ownership?
Which is probably always sort of implied, but not necessarily.