Fancy guppies. Nothing is easier to breed. Selling them isn't hard either. Lol. I bred and sold Neolamprologus leleupi in a 30 gal for a while. Was quite successful.
If you will also be using the 30g as a grow out tank you will have to do some serious culling starting at ~1 month. Nice thing though is you can leave the male in the 30g throughout the entire process.
A harem of the smallest variable Mbuna species would probably work and you wouldn't have to worry about the fry nearly as much. Africans have always sold better than SA/CA IME. There are plenty of smaller Rift Lake species that can be kept 1 male; 3-5 females if you got the skillz PLENTY in the Melanochromis genus that stay under 4"
If you are planning on selling back to the LFS, find out what they don't usually sell or can get cheaply at wholesale.
Those small 5 or 7.5G tanks are really useless for most cichlid fry or any fish other than nano species since water quality can go down hill rather quickly. 10G tanks are usually recommended for initial fry tanks.
Smaller Tanganyika cichlids would be a good fit for a 30B as would Malawi/West African that mature under 4".