There are several Petco's near my house, which all have terrible fish selection (marine)...
There is a mom and pop nearby that has a decent selection, but is notable more expensive....
One Petco out of the bunch is hands down one of the best I've ever seen. Wonderful selection, mostly healthy fish (even the mom and pop has some diseased fish and pest anemone problems)... this all stems from them having a few stellar employees who know what they are doing when it comes to maintaining the fish. The main employee, with whom I deal, not only takes care of his fish, he'll tell me to go to the mom and pop, and give me directions as to how to go there (even though i'd already discovered it on my own).
Due to the higher volume of sales, this store is able to purchase and maintain a healthier inventory of marine fish. The store chain simply will not invest in thousands of dollars of marine fish for one or two of you to maybe buy one twenty dollar fish. They'd rather just cater to the freshwater.
I'll buy from Petco if the fish is healthy and it is a decent price.
I also buy from the Mom and Pop if the fish is healthy and I like it, as it will always be more expensive... (sometimes more than double...)
FYI... Big box stores started small too... (even though I hate how big they've become)