What to be aware of when buying scallops; look first at their color. Scallops are naturally ivory or pinkish tan; processing (dipping them in a phosphate and water mixture to extend shelf life) turns them bright white. Processed scallops are slippery and swollen and usually sitting in a milky white liquid at the store.
You should look for unprocessed scallops (also called dry scallops), which are sticky and flabby; they will taste fresher than processed scallops and will develop a nice crust when browned because they are not pumped with water. If they are surrounded by any liquid (and often they are not), the juices are clear, not white. When in doubt, simply ask your fishmonger, although usually good markets will label unprocessed scallops as dry. Sometimes they are sold cut up, but best purchased whole.