By controlling the sex of what you sell as a feeder. I will sell either all males or all females, and people that buy males one day will keep getting males, so I dont accidentally create breeding groups for cheap.
The only reason I do this is because I BATTLED my wife to get them into the house in the first place, and the care and love that I have for them wont allow me to be undersold so other people can get in on my hard work for cheap.
Oh and remember, sunflower seeds are for when they're little, because it contains a lot of fat. You dont want them on it constantly or theyll get fat and unhealthy. I find that if they gain a lot of baby fat throug sunflower seeds, theyll use it to grow faster and will run it off playing around in the grow out tub.