If the fish doesn't look good, say it covered in Ich or isn't swimming right or breathing heavily, then the LFS shouldn't sell it, but certain people would take a chance on buying that fish at a reduced amount, as they think they could cure it and for the reduce price it maybe worth it. The same with fish that don't eat, a lot of fish come in and wouldn't eat for a while. Now the fish can be very healthy, it just doesn't take to the type of food that it's being offer. Some people will take the risk that they can get the fish to start eating. So depending on the situation the blame, if there is even someone to blame, could go to either place.
The question of whether a fish is healthy, just because it looks good and is eating, isn't always the case. The fish could have parasites or a disease that just isn't showing at the moment, and any fish that is coming in to a LFS, and is sold right away, or fairly fast, there is no way that a LFS can possibly treat that fish in the time that it had it, regardless of what and how the LFS is doing to treat the fish.