You have to reverse engineer this sort of thing. Check out the local independent shops; find what sells well, and what they are looking for in your area. The usual bread & butter fish are the best movers; livebearers like guppys, platys, and swords; n.w. Cichlids such as angels, discus, and some of the dwarf species; African cichlids depends on the species; your more common corys & plecs are the fish that move.
Next, find a local aquatics club, and figure out who is breeding & selling what. There is bound to be a deficiency in breeders of one of the bread & butter varieties, around me it’s L number plecs ATM. Target your breeding setup towards these species. Being in a club gives you a line on breeding techniques, as well as used equipment. Buying used gear, and building as much of your setup yourself as possible will reduce overhead.
Plan on making little to no profit the first year, put any money made back into your setup. Sell nothing but the best quality fish, bred or acclimated to your local tap water, locally bred & quality are the only reason a shop has to buy from you instead of a supplier or importer.
I see you are from Illinois, I know plenty of people around Chicago, and some further south towards Peoria & such. I deal with clubs & breeders here, in Indiana, and Wisconsin on a regular basis. Let me know where in Illinois your are located, I’ll see who I might know from that area. If I don’t know someone, someone I know is bound to, us breeders yak a lot.