I think it depends on where people are getting fish. I keep a lot of types of ricefish and have no problem selling them online and at swaps, but they don't sell in my LFS. We can always sell cories and plecos to our LFS stores. Nano fish like CPDS, clown Killis, and smaller rainbows are easy to breed and sell too.
Great question. We think of it all the time since our goal is to breed fish and then sell them. Sometimes we start a breeding project of something we find to be cool, but have a smaller market. Other times we can't make enough!