I get my fish the same day it is killed from the fishmonger. Properly stored, it usually has a shelf life of 7 days. Most fish (sold fresh in the grocery store) has been frozen and thawed. A lot of fish that is "on special" has been warehoused (frozen) for awhile. I assume the fish had an expiration date? If it was on special because it was close to it's expiration date, then you probably would have wanted to eat it up before that date. Portion wise, I plan 5 oz per person dressed weight. If it is a whole fish sold hanging weight, I have to factor in how much is lost when dressing it if I want the whole fish to figure out the # of portions I'm getting. I try to buy the # of portions I need (I portion it at home, but get a slab of salmon that weighs 10 oz, for example, if I want to make two adult-sized servings). In the future, I'd suggest asking some questions about the fish on special--where caught, when caught, when killed, if it was frozen (on the boat or on land), when the store got it, etc.
My fishmonger's prices are pretty competitive, as long as the fish is sustainable and in season. Here in Canada, the mark up on fresh fish in the store is 100-105% (based on the auction price--which is where our local fishmongers get most of the fish sold). Local fishmongers pay an auction # fee to attend the auction in Montreal. The ships come in with the fish in tanks, alive, one night, are government inspected the next day, kept in the tanks, sold the following day (auction starts at 0-dark hundred) and killed before they are packed on ice and refrigerated trucks bring them to Ontario. This is why fishmongers here are closed M-Tue and offer discounts of Sundays, that is usually fish Wednesday or Friday and should be eaten that day or the next.
If you have a local fishmonger, a visit when fish is on sale at the supermarket might be worthwhile for comparison shopping purposes.
I can't eat oily fish or fish that is too fishy...supposedly you can use lemon juice before cooking it to remove the fishy smell and salt the fish before cooking it to remove the oil. I stick with fresh fish and fish I know I can stomach. Smoked eel is not on my list, nor is fresh eel or mackerel.