Also, why would anyone she sell seashells by the seashore, of all places? Terrible business model to sell seashells where your customers can easily find their own.
From what what I remember, this saying is about Mary Anning and how she sold fossils by the beach in England where she lived. So most people probably didn't know how to pick a rock with a fossil, or open it well enough to preserve said fossil
So people say. I'm a bit sceptical about that origin story because fossils aren't seashells and, as far as I remember, Mary Anning didn't sell many of her fossils!