1) Your shells are really fresh if they can close tight themselves. Crabs and lobsters are not fresh if they are internally dissolved. You can ascertain this if you shake them gently.
2) A fresh and healthy lobster has its tail turned inside. A fresh crab must be in a container full of salt and ice.
3) A unique tip now. Spread many thick salt grains in a Pyrex bowl. Take a cleaned average (about 1/2 to 1 kilo) fish, put some aromatic herbs or spices in its stomach and put it on the salt grains. Cover it completely with thick salt grains and cook it in the oven for 45 mins. Then take it out, break the salt crust and put some oil and lemon on it. You will be astonished by its taste.