Steaming crabs is something that requires some special cooking equipment. Large tongs to handle the critters, and a very large steamer pot. Most homes don't have that. If you have one, great. Go to Old Bay.com and look up their recipe. Usually, it's an inch of water and vinegar in the bottom, a good handful of Old Bay spice in the liquid, a steamer basket to keep the crabs out of the liquid (steam, not boil), once liquid is boiling and steam is flowing, add a layer of crabs and dump a bunch of Old Bay on them, another layer, more spice, etc. Close the lid and let it go for something like 20 minutes (check the recipe). When done, crabs will be bright red.
As for buying crabs, they need to stay out of water and out of the heat. All day is quite risky. Usually, you want to buy them and bring them home to cook, not sit around. They die easily and at $3-$4 per crab, that hurts. They will live a few hours if you can keep them in a very cool place like the refrigerated trucks that you buy them from. Never ever cook a crab that's dead! Very important. You will get sick. When you grabs the crabs with tongs, make sure they grab and grip the tongs to show they are alive.
Oh, and many places just put them in a few paper grocery bags. They're so busy grabbing on to each other for dominance, they won't try to leave the bag. They really are nasty tempered things.
Edited: 17 June 2012, 2:01