For anyone in the Philly area, check out Samuels and Son

They're a major seafood purveyor for restaurants as far away as Pittsburgh and the fish is as fresh as you can get sans off the boat. Navigating the web site is a bit of a PITA, as it takes several windows to get to deal of the day/month. Call ahead and order, otherwise it takes a while for them to go back in the warehouse and fill it. You also have to buy in certain quantities such as 50 oyster minimum or a case of a particular item.

Ippolitos is their retail outlet, and is close by. Prices are about 15% higher and the selection isn't as vast, but it's far better than most markets. Service is much quicker and there are no minimums. They also have a good selection of prepared food.