There's a great book, "So you want to start a nursery" Get it. Read it.
1. Don't get nursery mixed up with garden centre. They are really two different operations. One grows plants, the other sells plants.
Few garden centres grow all of the plants. Many don't grow any. They buy ready to sell plants, and then do their best (feeble at some big box stores) efforts to keep them alive and healthy.
Most nurseries grow only a few categories of plants, often aimed at a particular niche. A few outfits do both. I'm one of them.
I ignore the non-plant side of garden centre. You want fertilizer, hoses, etc, go elsewhere. I could do that. But it would mean another building, another staff member.
I figure I shouldn't grow anything unless I can move 300 of it a year. Sometimes two somethings are similar enough that it's worth doing anyway. So at present I grow 11 kinds of poplars, with quantities ranging from 50 to 2000 a year.
Things like fruit trees have increasing interest, but I only sell 10-20 of any given kind a year. Buy them at 7-8 feet ready to bloom, repot the surplus 1 pot size higher at the end of the summer. This allows me to carry about 40 cultivars.
2. Learn about automated watering system, and use them. Most trees & shrubs you buy from wholesalers are in a very light weight bark based potting mix. Water runs through it fast. depending ont he plant and the mix you may be applying 1/2" to 1 inch of water daily. Watering MUST happen. If the truck breaks down across town, and there is a 6 man crew sitting idle, but the next sprinkler change is in 45 minutes, you tell the crew to call AAA.
3. Don't try to compete with Walmart. Sell other things, or at least in different sizes. If they are selling 1 gallon junipers for $4 bucks each, buy a bunch from your wholesaler, pot them up in #5 pots and grow them for 2 years, and sell them for $40 each. On the otherhand, if Home Depot is selling 8 foot arborvitae for $27 each, see if you can go after the seedling market and sell 2 yr old rooted cuttings for $20 / 10.
4. Be a font of knowledge. if you don't have it know who does. Track those 'don't haves' and consider adding them to your line.
5. Find out what is the most common, and specialize in having a better selection and quality. E.g. If Home Depot has 6 kinds of juniper all in 2 gallon pots, you have 25 kinds ranging from 1 gallon to 10 gallon.
6. You can find out what sells by parking outside a big box store, and watching what people bring to the till.