why can you find, say, sharks teeth on some beaches but not others or sand dollars or conches etc?

Answer 1

Sharks only like to fight on certain beaches. It's part of the Shark code.

Answer 2

The animals that populate the oceans have their own territories, just as land animals do. So, for the same reason you don't find tigers wild in America, you won't find certain shells in the water off certain beaches. The water may be too hot, too cold, too salty, not enough, etc. Even the ocean currents aren't enough to move these shells from one beach to another without destroying the shells in the process, so you usually will only find local shells on your beach.

Answer 3

Tides and Water Currents.

Each beach has different tides and currents, and so some beaches will have some things, while others wont.