It may seem hard, but raising and breeding birds for profit is not that complicated, although it is much work. You need to be able to devote 24 hours a day and 7 days a week to birds if you keep them and want them to breed.
The budgerigar is one of the best birds out there to breed, they are particularly good for amateurs and those who are just starting out with being a owner and breeder, because of their size, they are quite small, and low level of noise. Many males learn how to speak and they are like created for smaller spaces, apartments and flats, because they also don't need to much space. You should take a DNA test before getting into breeding, to make sure the two birds you have chosen are good enough and that they can go further. It is also important to have the right temperature during the whole day and night and get a handbook, it will help you much better, because there are too many things to know.
Cockatiels are also quite good for breeding, they are also small, but even the advanced breeder tend to stick with them, because they are always wanted and they are so sweet. Their vocalization is quite fun and pleasant and their personalities are special, those two things make them so popular. One more thing that makes them so special is their huge range of colors, they can be found in almost any of them and that is partially to be thanked to genetics. The parents often take big care of their chicks until they are sold. For aviculturist they are the perfect species to start with and consider not breeding them indoors, because they generate a big amount of dust, so your best friend would become your vacuum cleaner.