Hi! "Pesce" is a great place to buy fish and seafood: Good quality and prices. It's in Vicente Lopez. http://www.pesce.com.ar/index.php They have a delivery service.
I buy scallops occasionally because I love them and they're not bad, and the certainly won't make you sick but I've yet find "great" ones. Decent yes, I buy mine from the pescadaria on entre rios between chile and mexico
The markets in Chinatown in Belgrano are the best place to buy seafood. Chefs from many of the best restaurants in town buy there. They do have frozen scallops , and they are real , and good.