I guess, to qualify my statements, none of my LFS's really try to gouge on prices. Many/most prices are in line with J&L plus shipping with the convenience of shopping in person.
I agree that if my LFS was charging 40% more for salt, I wouldn't be shopping there either
I don't own a fish store, but the assumption that they make there money on livestock, I think, is wrong. You have shipping costs of water, it's heavy. You need salt in the water, equipment to manage the water, tanks to put the water in. you have to feed and light the livestock, so food, bulbs, etc come into play. Things die, that has to be absorbed by the remaining stock. It takes more staff time to take care of the livestock vs. dusting the drygoods.
At the end of the year, I don't think it's a huge profit margin.