the best shot for a small beginner nursery is to sell what local nurseries do NOT sell. They will definitely outcompete you with prices and customer volumes, to be recognized you have to have something different, so the best shot is to concentrate on niche plants, more rare varieties, variegated/weird ornamental plants that are just coming to be in fashion, or rare fruit/palms/whatever that cost more than regular varieties but are also harder to find, so people are hunting them down. For those you surely need to know the variety, as same species can have a $5 and a $500 plant standing next to each other...
Rare plants also require more expertise for growing so make sure you know what you're growing and each plant's water and nutritional requirements. Some plants don't take regular soils for example, you have to make special mixes; some plants need full shade and wet air, so you have to have a special shadecloth/misting setup in your nursery which will also require an extra expense.. Join facebook groups like plant hunters, time to splurge and purge, Jumanji Jon plants, Ralph Plants, etc. and see what's trending. Known rare plant sellers sell their plants out like hot cakes, sometimes within minutes, if the plant is popular - even a single bud on a stick (a chonk) sells for a lot of money! This happened with lots of variegated plants recently (during covid), people just went crazy over them and paid up to several thousand dollars for one plant...
If you don't want to have a large initial investment, start with semi-rare plants that propagate fast (normally expensive plants are slow growing lol), or even create your own niche market, like you would sell 250 varieties of sanseveria, or only funny looking succulents like the boob cactus or the penis cactus... and go to those specific groups where people are looking for those specific plants (there are groups for succulents, palms, sanseverias - basically any type of plant if you're willing to ship). I think with selling plants it's more about your personal passion, plants that you really love that you will be able to sell the best and care for. Good luck.