Depends on the fish. Some pet stores will gladly buy them once they are 1" or longer if it is something they want. You could also ship them if there was a demand. I'll probably grow out a batch of Festae fry once they breed as many people in this area have already shown desire for them.
I breed now mostly for food reasons. When breeding your own feeders, you have a much lower chance of introducing disease into your tank that may have gone unnoticed at the LFS. My convicts supply me with a steady stream of offspring; you can't give them away if you wanted.