Bettas are actually quite difficult to breed. You first have to keep them separate and condition them, then bring them together very carefully, then separate them, then be prepared to separate fry as they grow. And the fry need small fry food.
Livebearers are easy- the problem is that everybody's doing it. Whether they want to or not. So there's a glut on the market. Unless you go for one of the more unusual livebearers. The common ones are mollies, platies, guppies and swordtails. Slightly more unusual, but very popular are endlers. Then you get a whole range of unusual ones like black chins, portholes, merry widows, heterandrias etc. And the goodeids which form a separate group- and need a bit of extra tlc as they are threatened in the wild. Also halfbeaks- again, I think need a bit more specialist care.
You might prefer some of the easier egglayers. Cherry barbs, corydoras, danios (though you won't make much money on the danios).