"Az eladónak fehér ruhája van."

Translation: The sales clerk has white clothes.

[gr8fuld34d]

Could this also be translated as "The sales clerk has a white dress"?

[vvsey]

Yes. And it can also mean that he is wearing it/them at the moment and we are looking at him.

[guntunge]

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.