the majority of locally owned shops prefer to get locally bred fish, some easy to breed fish just aren't practical for shops to buy... examples include livebearers (unless you're dealing with some impressive strains) CA cidhlids (the market for them is too small to buy any number of them on a regular basis) and Bettas (are too expensive to raise and come dirt cheap from major distributors)
Fish that are good to breed for shops include cory cats, angelfish, discus, killifish, or the big one I'm surprised more people don't get into is glofish, the genetically modified danios. Contrary to popular belief they are fertile. It is technically illegal to sell them for profit since their genome is copyrighted, but there's nothing preventing you from exchanging them for credit.
If you're thinking about breeding SW fish be prepared for a tremendous investment of time and money, the required facilites to raise the fry to a sellable size take up about 200 square feet per species and cost at least 150K to set up. If you've got more cash than I've ever seen in one place burning a hole in your pocket and access to cheap climate controlled warehouse space breeding clownfish might be a great side project for you.