I'm sorry to rain on your parade here but stores are looking to make a profit and the more so the better.
In my experience the LFS will only offer you around £1-£2 (if your lucky) per fish. And they will normally sell your fish on for around £6-£8 per pair species dependant. I know its not much for all that hard work.
And killifish are notorious for skewed sex ratios. The way I do it is to negotiate a food deal. frozen or dry goods, you may well be able to get a better return on this type of barter. If all else fails search out local aquarist clubs, they will have auctions at various times throughout the year.
Always remember fish breeding is not a get rich quick scheme, Gawd knows I've been doing it for years.
Selling eggs would possibly be a better option as you personally dont have to worry about the skewed sex ratios or the feeding to get your charges to a saleable size.
Expect to sell 18 to 20 eggs for around a fiver and you will have to factor in your postal charges. possibly £1.50 if done correctly in the summer months(padded bag and insulation) and more if you include a heat pack during inclement weather.
Always include instructions on the species in question, when the eggs were collected and when they are expected to hatch.
Wait until the eggs show sign of development before sending, that way you earn a reputation of selling viable eggs.
Hi big c im looking to have a go with hatching some eggs and growing on killifish, im assuming your profile picture is of some you have, do you have any eggs of that type that you wantto sell? cheers mike