I think that there's some cost involved with breeding fish, especially bettas. Even if it is livebearers, which breed prolifically, you would still need extra tanks, sponge filters, heaters, liquid test kits, etc. It's good that you are researching before starting up this project. Do you know what kind of fish to breed?
I see from your aquarium information that you don't know about the nitrogen cycle. Please read the link in my signature to learn about it - it is imperative knowledge if you want to run a successful fish business. Also, I see that your water parameters are all zero. Since you don't have live plants, that means your tank isn't cycled. I don't think you can successfully breed fish without a cycled tank.
I started selling my fish online 6 years ago. I have 138 tanks mostly cories. I sell at auction sites such as aquabid and foroum site like this. I have not set up a website etc. because I have a reguler job also. I sell about 300 $ worth of fish a week on average. If you work out the cost per hour I might make a $5.00 per hour for every hour in the fish house. Great part time gig and with a little effort could easily become a decent buisness. Kens online fish food etc is a great example. He started with selling some food from his garage to help when he retired. He now has 6 people working for him and is one of the best if not the best online fish accesory stores.