I wish you luck! What a wonderful idea, someone who loves the hobby setting up a store. A refreshing change from all those employees out there who don't know a dying fish from a healthy one.
My advice is this:
1. keep your fish healthy. Quarantine new stock for at least a week before selling. If fish become sick, treat them immediately, and don't sell fish from that tank.
2. If you import wild fish (please don't, but if you must), please make sure they're not fish that will waste away in captivity. I was at a petstore today and saw two SW fish on their last legs, so skinny I didn't know how they could still be alive. I can't be sure, but I think that one was a species that only eats specialized organisms off coral, and it was living in a bare tank.
3. Decorate your tanks! Stores with rows and rows of bare glass tanks stuffed with fish are not only boring to look at but unhealthy for fish! By putting the proper substrate / hiding places in the tanks you are showing the customer not only that you are knowledgeable about the fish's needs, but also educating them as to how to house the fish once they get it home! When I bought my first pleco many years ago I didn't know they needed driftwood; they lived in a bare tank at the store!
4. You might consider setting up your display tanks with different filtration systems, so you can show your customers what they look like in place, and illustrate the advantages and strengths of each.
I applaud you for collecting ideas and doing your research, I have no doubt your store will be a success.