If I were exchanging emails with the seller, I would ask precisely how the portraits were made: i.e. from what negative, obtained in what way. Obviously the original negative is not being used. Mostly likely, the negative (if there is one) was produced from a scan by the LOC.
As a preservation measure, we generally do not serve an original item when a digital image is available. If you have a compelling reason to see the original, consult with a reference librarian. (Sometimes, the original is simply too fragile to serve. For example, glass and film photographic negatives are particularly subject to damage. They are also easier to see online where they are presented as positive images.)