You need to be ready for two distinct type customer although there may be many, I see 2 a lot.
Type one; fairly new to fly fishing and impressionable. For this person you need the brands and products that are featured in current marketing campaigns. The new & sought after rods & reels etc. These folks will have plenty of questions along the lines of, "so what's better", and you'll have to be ready with the answer.
Type 2: These are people who have everything and are browsing your inventory. For them, hard to find hooks and tying materials will be a find more often than a new rod.
I'm a type 2, I like finding a few spools of silk in a color I don't have or some neat hooks to tie on. My most recent buy were several packs of Senyo Shanks to learn tying articulated flies on. The guys at the shop I go to know I have a bunch of rods & reels but are always ready to show me the newest thing in fly design.
Ard nailed it. I live about 2h from our closest shops and there are two in the city. The first has the usual tying stuff, but the second has some unusual and hard to get items. I always go to the second shop when I go to the big city, and rarely go into the first one because it is the kind of mass market tying stuff that most shops carry. I can get those materials online and save the 2h drive, but the second shop has some really hard to find items.
IMO, there are 2 kinds of shop. One has all you need for the local fishing, with very limited inventory for other areas, while the other has the range of inventory to cover any destination. The local shop usually has specific water nearby that they specialize in, and may also have guide services for those local waters. The better shops also have quality people, who can give good advise no matter what the type of fishing folks might be interested in doing. Nothing loses customers like BS. You can get that at the big box stores.
The full range shop may also have guides, but they'll be more of a reference to outside services at the destinations.
IMO, the best shops cover a range of equipment needs for the type anglers that Ard mentioned, and will have a full range of other items, such as tying materials.
The unfortunate thing about fly shops is none will carry everything. Most folks who own shops don't have the finances to do so. Also since fly fishing is such a niche sport, it's difficult for such shops to survive during the off seasons, unless they do things to keep their customers coming back during those off seasons.
I had a mail order/internet business for many years, and would have loved to owned a brick & mortar shop, but knew that it was not viable. I primarily tied flies & sold tying materials, but never made enough money with it to make a living. I also ended up making lures, which increased my customer base somewhat, but in my geographic area, once we got into the colder seasons, fishing stopped & hunting became the sport of interest. I didn't sell anything hunting related & most sales I had during that period was to out of state customers in warmer climates.
I also taught some fly & jig tying classes, particularly in the colder months.
You have to be very diverse, and willing to put in a lot of time to keep a shop operational. Don't think you'll spend a lot of time fishing either, unless you have good quality help to run the shop.
For us, its all about a large non-homogenized inventory. Good shops carry all the standard brands, but great shops carry the rest.
We can buy just about every brand of rod you can think of here, but you can’t walk in and select a blank, reel seat, stripping guide, hook keeper or cork rings off the shelf….same goes for many of the curious ( fun to own and fill out a collection reels, etc. )….can’t remember the last time I saw a Valentine planetary reel or tying vise other than the usual suspects.
All the shops here, pretty much parrot each others inventory…it works for them and the type 1 anglers Ard refers to, but it doesn’t work for us, so they don’t get the sales from most of our gang.
The people running the place set a great shop apart. All the inventory in the world doesn't make up for knowledgeable, friendly personnel. I go to one shop locally all the time because they like to talk about fishing and are nice folks and pretend to like me. When I take my annual vacation in Colorado I visit a shop there every day because those folks are fun and great people to talk to. This is the most important aspect of any shop to me.
Above all, I agree w/ dwtalso. The people that are in the shop set it apart. If the people there are the type who want me to buy something or leave, I'll be damn sure never to go in there again. But the guys who BS with me for an hour when I stop by either to buy something very small or just to stop by they will always get my business first. These are also the ones who will do whatever they can to get you what you want if they don't stock it and give you all the advice you need.