Part 2 of my 7 saltwater fishes you should never get feature the Elasmobranchii class of fish. Elas- for elastic, soft, and branchii is for the structural part such as the bones. Basically it means fishes with cartilages: Sharks and Stingrays.
Some of the more commonly sold sharks and rays in the aquarium trade are bamboo charks, cat shark, nurse sharks, blue spotted stingrays, and fiddler ray. Many of them are sold as extreme juveniles or even eggs (shark). In the case of bamboo or cat shark, one can make a case that many are aquacultured and does not disrupt the marine eco-system since they are not harvested from the ocean. However, from a humane stand point, I personally would not feel justice to obtain and maintain a healthy shark in the cat shark family unless I have at least 2000 gallon in water volume with plenty of space to hide and to move around.
Lastly, even though they have the label of being ferocious predators, they are not garbage disposals that will accept anything you place in the tank. The store I used to work at had ordered some cat sharks and bamboo sharks in attempt to sell them. No one ever actually bought them but they do act as a good customer draw and conversation starter. Unfortunately even with the large system and clean water condition we weren’t able to get the little guys to eat. They require some really sophisticated mix of natural/live seafood (clam, oyster, shrimp, crab, fish cutlet, etc)
And even if you can manage to keep them alive, they may outgrow your average tank (if it’s anything under 300 gallons) that you will need to give them up.