The very best way to buy scallops is live in their shells, this way you can guarantee they're freshness and taste. Scallops are easy to open: Wash them to remove any sand, place the scallop flat side down on a board and simply run a thin sharp knife between the shells running it along the bottom shell, keeping it as close to the shell as possible. This will kill the scallop and the shell will open, carefully remove the scallop from the top shell with the knife. Trim the scallop leaving simply the tender white meat and the coral (optional). Of course you can ask your fishmonger to do this but it means that the scallop is killed and will need to be cooked soon as the flavour will quickly deteriorate, I like to open them just before cooking.