Such bulk purchases of books by political figures are quite typical, Left and Right. Certainly there is no other way to account for Hillary's book sales.
Politicians publish ghostwritten books because book sales, bulk or otherwise, are not subject
to campaign finance restrictions. Individuals are limited to only a few thousand dollars
contributed to a candidate, but can buy any number of books. I suspect a very small percentage
of politician-authored books sold are actually read, or even removed from their cartons.