I was hoping the article would have answered how the Prince (or his buyer) even heard about the book? I wonder if it was recommended by the shopkeeper, or if the fact that it was written "By A Lady" was enough to pique his interest, in the hope that it would be more salacious?
They seem to imply that he had buyers who would have sourced the book for him. I'm guessing that the buyers had contacts at the various publishers so they knew in advance when interesting new books were coming out.
This is an interesting article, but to be clear: this is about Sense and Sensibility, which isn't her first novel, so "First Buyer" seems a bit of a reach.
Getting a book before publication definitely sounds like the kind of thing you'd expect the rich to do, however.