If you are planning a breeding project that involves genetics, you are going to have a lot of fish that you don't really care to keep (or at least will not further your project anyway) so you will need a plan for them,I don't think I said anything about 'selling' necessarily. I just want to create a thing of beauty that others would be able to appreciate. From the looks of the landscape things once thought 'common' may not be so taken for granted as of late. This should be easy but it's not.
Additionally even if I was trying to sell the exact price would not be as important as the margin and quantity sold, but again, this is a hobby for me. My professional occupation is something else entirely, and pays well enough.
ask anyone who has bred convicts, or bristlenose, or non-show guppies they will tell you how difficult t is to even GIVE fish away.
you may want to say specific what species of fish you want, rather than just listing the genera 'danio' . I want to say that Zebra or Leopard are fairly commonly sold as 'long finned' but you may be thinking of something else.
if you are trying to experiment with genetics, and want to keep the footprint of your operation down, I would suggest maybe looking at ornamental shrimp rather than fish. If you not worried about your number of gallons of water, I would look at angelfish or discus for their popularity. If you want to look specifically at line breeding and to be able to see lots of variety, and where the genetics are fairly well documented, try Betta splendens or guppies. There's always the option of other established live bearers, like swordtails, platies or mollies etc.