All excellent suggestions so far. I think the most critical are location and staff. Most gun shops in my region always seem to be located in crack town in run down facilities. You feel like you need to be carrying something belt fed just to walk through the parking lot, judging by the surroundings.
Staff is #1 though. As other posters have mentioned, a bunch of cranky bums sitting rocking chairs that are seemingly annoyed when customers walk in is a real turnoff. A staff that will take the time to determine what a person's requirements, needs and wants are, possible physical limitations (ie, some people with small hands and short fingers are probably not going to be able to operate a Glock well, which seems to be the answer for everything from some gun store commandos), then offer up several different options either in the display case or that can be ordered for cost + X%.
Communication is another biggie. If you order a gun or accessory for a customer, you will give the customer an estimated arrival date (like most anyone else would do). Note that date on a calendar, computer, smart phone, etc (something) along with the customer's name, phone # and email address if applicable. When that date pops up and John Doe's Ruger Blackhawk didn't show up, be proactive and find out why and provide the customer with a new estimated arrival date, without them even having to ask. Rinse and repeat. Trust me, having to track down the status of a late order is something that cheeses a lot of people off. Stay on top of it.
Selection is very tricky, particularly when just starting out and buying the inventory on credit. My plan would be to acquire the inventory with cash, even if it means starting a little smaller or delaying the start up. I've seen new shops struggle because they had to build credit up with the distributors and until they did, they were charged a higher markup which tended to make them less competitive against older, more established shops in the area. Talk to those distributors and find out what you can do to make yourself more effective (ie cost competitive) so that you can be successful.
As for the inventory itself, ideally you could stock everything. But unless you are loaded with cash to start, that probably isn't realistic. So you'll need to stock an inventory that aligns with the demand in your local area. I can't say what sells in your area. In mine, lever guns sell better than any other rifle class, so that is what you predominantly see. And because I live in a region with a distributed population, there isn't a lot of places to shoot .50 cal rifles (unlike the western US) and you only seem them in a store as perhaps a novelty or a marketing tool. Single action Rugers are hugely popular with handgun hunters around here, so you tend to see a lot of them with different scope and mounting options around here. But your area may be different.
I guess you have to ask yourself, and others, what people like to do with their guns in central Mississippi.
