Arrow crabs eat snails, feather dusters, smaller inverts, and when they get larger, can even eat small fish. They are not something I would want in my aquarium, however, they are still able to be kept in captivity in the right system.
Dragonettes are another fish that are very keepable in the right conditions. Many things just come down to researching before making a purhcase and ensuring that you have the right tank specifications, water quality, and tank mates to go along with whatever you are looking to buy.
Ophiura, thank you for the clarification about the starfish, and the inclusion with the feather stars. I totally forgot about them because I have only seen them for sale a few times.