I do love mussels in red or sauce. Also how will I know what fish/crab is safe to eat?
fish filet should look bright, not feel slimy, and should NOT smell like fish. You are in Philly, a pretty fish friendly place, as the food scene there supports the need for higher quality, fresh fish. If looking at whole fish(small tilapia, milk fish, etc) they eyes should be clear, not milky, the gills nice and red, not dark or brown, and again, should not smelly fishy.
Frozen Filets are readily available in most markets, and great deals can be found in the Asian Markets. Farm Raised Sea Bass, Sword, Mackerel, Mahi, salmon, Snapper. . . all should be readily available in single service, IQF portions.
Crabs, if buying live, they should be very lively, kicking and clawing. Crab more along the lines of snow crab, king crab, just have to pass the sniff test too. They should smell sweet, and like the sea. . . again, not fishy.
As mentioned, Mussels are great, and cheap, and if you like them, there isn't much to cooking them at home. Just make sure you clean the beards off, and clean the grit off the shells. Toast some garlic in some olive oil, add a pinch of dried oregano, parsley, thyme, a few capers, crushed red pepper, add the Mussels, add a splash of whit wine, cover, steam until opened, add a little tomato sauce, a pinch of salt, let the liquid come down some, add a knob of butter(optional), or finish with olive oil. Serve in a bowl with some crusty bread. Might set you back $8, total.