Best bet for that is aquabid. Pick up some smaller fish, grow them out & sell for a profit
I breed angels, a year ago the market was swamped. I was getting small angels at an auction for 70 cents each, which means mine sold for the same. Got some different varieties that I was not breeding at the time, grew most out, sold them at dollar body size for 5 to 8 bucks each. Kept the best for breeders, made a few bucks, and am in the process of setting up tanks for a lot of new pairs.
As far as culls, they are non-saleable fish. Any responsible breeder will not let them leave the house as anything but feeders. I used to give them to a friend with an oscar. His fish died, so I had to get my own oscar. Culling is the least enjoyable aspect of breeding, but a necessary evil. It's easier to look at it as more of a quality control issue, if you can't offer the best quality, your sales will suffer.