This may sound harsh, but most people who have spent a little time in the hobby will discover that they can get more money (or any money) or trades for either Endler's (P. wingei) or guppies, but much less for a cross. In effect, when someone tired of them, the crosses are culled and dumped in the feeder tank.

I once stupidly put some killie fry together in a grow out tank, thinking that they were the same line. (Note to self - do a better job of labeling tanks.) When they began coloring up, to my dismay, it became obvious that they were two color morphs of the same species with virtually identical females (Fundulopanchax gardneri Misaje and Nsukka). With a really heavy heart I gave them to a local shop as feeders.

At fish club auctions generic (aquarium strain gardneri) sell for 50-25% of what the pure strains sell for. It sounds a little snobbish, but as aquarists get more involved in the hobby, but still have limits on their space and time, they become a little more picky about what they buy.

That is not to say that they still do not try for crosses - curiosity is a hallmark of someone growing in the craft. They just learn not to indiscriminately spread around those crosses. It should be a controlled experiment within a set space. While guppy x Endler crosses and some of the mollie, Xiphophorus and some cichlid crosses certainly are fertile, it is a dirty trick to sell them as pure species or strains to some unsuspecting newbie.