I would think that usually a breeder will trade one snake for another snake that is needed for their breeding program. You may be heavy with albinos and need a lesser, so you make a trade. I can't speak for others, but I've tried my best to buy the best example of the morph that I want to breed, so there would be no reason to trade for the same morph. But since I'm just getting back into breeding, I haven't found the need to trade any of my snakes yet.
Jim Smith