Every market will be different. Every store will be different. We retail Endlers at 4.99. Most customers only want males. Also the easier something is to breed the more competition you'll have. Routinely someone just drops off 50+ endlers at the store for free because they are crowding their tank.
You quoted me saying operate like a business. But you also asked, do I need a business license. You're already on a path to no operating like a business. You need a business license. Any store wanting to do actual volume, needs you to have a business license. This allows them to properly account for them tax wise. You'll likely need to store their wholesale/resellers permit on file to be able to sell to them without charging tax. Both parties will want invoices so that they can deduct the items from taxes. You'll want to justify writing off foods, electricity, partial rent, tanks etc. They'll want to be able to show it as a COGS and deduct it from the profit.
Just because a store isn't selling something doesn't mean there isn't a market for it. I don't sell lots of stuff, but it's because we focus on a specific thing, planted community aquariums.
My best advise would be get started, learn along the way, run a legit company. Every time you step away from a legit company you're 1 step closer to just being a hobbyist. Get a business license, get a checking account and credit card in the business name. Buy everything with the credit card to make tax time easy. Then take cash, check, credit from the stores, with invoices. The easiest will be paypal invoices/credit cards at the beginning.
I don't mention this in videos, but lots of people are never meant to be the face of their business. People assume that's how it works because they watch me do it. But most companies have lots of people who aren't good at selling, but great at running day to day operations. You may be terrible at getting stores to buy, but a great breeder. Or vise versa. You'll need to learn where you struggle and improve on it. Often times you may have to create and supply marketing materials for the stores about your products. Theres a reason they don't sell it, could be education, could be marketing etc.
My advice is to start with something that's already selling at the stores. If they sell 50 red platies a month. And sell 0 endlers per month. Which do you think will be easier to get them to buy from you and have them sell volume? As you learn to supply necessities you can gamble some of your time/profits on what you think might be missing from the market.