First thing to ask is how much time and effort you are willing to invest, and how sustainable do you want to be? You can make a few dollars selling off guppies, bristlenoses, etc. from a couple tanks you happen to have. But for the time and effort you'll likely to be better off getting a job at McDonalds.
You can make more if invest a few thousand in tanks, racks, a room . You're going to want to breed a selection of species, and you'll need to deal with shipping. Selling locally you can easily saturate the market before you make a profit.
Very true. I had a buddy with constant fry in the tanks and every fish store in the area with his fish. It was to the point even the fish stores were having problems selling off that many fish. The low point was when I saw the fish store running a $1.99 special on his fish... and these were F1 red devils from big lipped parents.