Breeding fish is never as simple as it seems. If it is easy to breed, there is seldom a large demand for it (most livebearers, convicts, etc.) and if its worth it from a financial standpoint, its usually a lot of headache logistically with housing, water, and feed to all consider.
If you have your heart set on something fairly low maintenance to breed that will have a fair ammount of outside interest in it, look into some of the color pattern specific livebearers (mollies, platies, swords, endlers, guppies). IF you do it in a targeted fashion and not willy nilly, you can get some quality fry that an LFS might interested in.
Other things that could interest you might be some of the more basic freshwater shrimps. There always seems to be an interest in good quality yellows, blue pearls, and snowball shrimp. Also breeding CRS and CBS can be fairly easy as long as you have quality water, a good maintenance plan, and patience.
Wait, that is key to all of it. Patience. No quality aquatic stock will be quick and easy.