"Had you ever seen it before" implies you have now seen it, but not previously. "Have" does not carry that implication.

"¿Habías visto antes" implica que ya lo han visto, pero antes no. "Tener" no tiene esa implicación. ¿Alguna vez has visto? te había visto alguna vez? Yo lo he visto. Lo había visto. He tenido que ver. i había visto.

the when that person being asked have seen

"Had you ever seen it before?" is in the past, it implies that never saw before but now you are seeing for the first time. "Have you ever seen it before?" is in the present perfect, it means that you didn´t see before and you continue now like before .

There isn't really too much difference between the two apart from that "Have you ever seen it before?" would make more sense and flow more compared to "had you ever seen it before?"