Lobster and Crab
You should make sure that their eyes are bright and clear. Lobsters and crab should be alive when you buy them - never buy a dead uncooked lobster as you don't know how long it has been dead and bacteria spread rapidly in dead crustaceans. They should be lively and move their legs and claws when handled. A limp lobster or crab is close to death.
A live fresh lobster or crab should feel heavy to handle and seem heavier than it looks.
Mussels and Oysters
Mussels and oysters should be alive when you buy them. Live mussels and oysters will have tightly closed shells. Shells that are open should be discarded.
Scallops
If buying scallops in their shells, make sure they are alive by checking that the shell is tightly closed. If buying scallops that have been shucked (opened) they should look nice and plump and moist.
Prawns
Prawns are usually pre-frozen. Go for prawns from cold waters like the Atlantic rather than warm water prawns from Asia as they have far more flavour.