Question

if sally sells seashells on the seashore then who is buying seashells there literally all over the beach

Answer 1

If you've ever been to Sanibel Island, you'd see this in action. Sanibel is known for its seashells, and the beach is covered in brightly colored, large shells. People are walking along the beach collecting them. Everyone gets into a seashell collecting state of mind. As a result, there are tons of shops selling seashells, too, and people are buying them. The ones you can get on the beach are nice but the shops have the nicest ones, like giant conch shells, etc.

So even near the beaches with some of the most plentiful and nice shells, there are shell shops -- in fact, the higher quality of the shells on Sanibel's beaches seems to have directly led to many more shell shops than you'd find at other beaches with fewer shells.