Crabs are sold either live or cooked. Buy live crabs from reputable sources as they are highly perishable. Live crabs will have their front pincers tied with string, they should look fairly lively and feel heavy for their size. Never buy a dead uncooked crab.
Cooked crabs are also highly perishable, so buy with care. Make sure they smell fresh and are undamaged and their legs and feet are drawn into the body (if they were dead when cooked, their legs will be looser). Crabmeat is also available frozen and tinned.