Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
Congrats on the the fry! (And my condolences on the male.)
Breeding livebearers is really easy. Even in my 46g, with predators and large filter intakes that can easily suck in fry, and no special feeding, it seems like one or two somehow make it to maturity. With a little help, you can soon have more than you can handle.
Fry appreciate extra protein, so algae (vegetable) chips aren't the best. As far as what you're likely to have on hand, standard flake food crumbled to fine dust will successfully raise fry from day #1; although for the first couple of days, a tiny bit of hard-boiled egg yolk mashed to paste with a bit of water and dropped in is better. Try to feed smaller amounts multiple times a day; 2x daily is good, and 4x daily is better (they have tiny stomachs which fill and empty quickly). To give fry every possible advantage, there are plenty of other specialty and live foods, though they aren't absolutely necessary.
A lot of that food remains uneaten and can dirty up the tank (especially for the egg yolk), so I switch to 2x weekly water changes.
Raising tank temperature will accelerate their metabolisms and make them grow faster (and require more food). Pro breeders often go as high as 82°F, I usually go no higher than 80°F.
I try to get them out of the cup (or breeder box) just as soon as they're big enough to avoid predation. They seem to grow even faster then, probably because they can scavenge for the natural food Chicken mentioned. The filter intakes remain a threat in my tanks, so a bit of tulle (fine plastic mesh) from the craft store wrapped around the filter intake and secured with zip ties keeps them from getting sucked in. Keep an eye on it and make sure it doesn't clog.
Pretty much any guide on breeding guppies you can Google up will have additional tips that you might find useful.
As for sale price, it depends. Pure-bred Endlers with a known, uncorrupted lineage can often fetch a nice price. Don't represent them as pure-bred Endlers if they're not, or even if you're just unsure; call them hybrid instead.