I would not start with bettas, it’s not very fun getting stuck with a bunch of males that no one wants. Unless you have a nice pet store that is willing to take them I’d try another species first. Plus bettas are by far not the easiest to breed and raise.
I learned to ship fish safely and that’s were I make, not much, but a little money. Problem is some species just do not sell well unless you have higher end stock. Right now I’m having a hard time selling guppies and bettas (surprise surprise). BN plecos with long fins and more fancy color forms (calico, super red, albino, green dragon etc) sell really well especially if you can sell them with a bit of size. Rainbows also sell well, particularly the nano species. Corydoras sell well too, just pick a more popular species like sterbai or even dwarf species. I’ve had no luck with corydoras though.
All around best way to sell fish is getting friendly with a pet store owner. I’ve not had much luck as of yet. You can also sell on here once you get 50 posts, ebay, aquabid etc but only if you are willing to ship.