LOL, I don't sell my guppy young, I give them to the fish store for free! He sometimes knocks some cash off the total of things I buy or gives me a good deal, but it's out of kindness rather than a need for my guppies. Guppies and Endlers are cheap and mass produced, don't cost a lot, so stores aren't going to pay a lot for guppy/Endler young I'm afraid, unless it's a special fancy variety that they can sell for a higher price point.
My guppies are plain old farm bred store mutt guppies, not a fancy variety, so I need my LFS to take the hundreds of young they produce far more than he needs them as stock, since he can buy farm raised guppies for cheap in mass quantities at any wholesaler. It costs way more to raise them to 2-3 months old than I make back in store credit. Breeding them has just been for fun. I need him to take them since they breed so easily and in huge numbers, I'd run out of space quickly, and selling them privately is a pain, organising to sell 2-3 fry per person for a £1 or so each. Not worth the hassle really for me to do it privately. While if the LFS stopped taking my guppy offspring, his business wouldn't suffer in the slightest!
Yours are very pretty, and I can't believe they were in a bathtub in the woods! What a horrible person to dump them there
Happy that you found them and are giving them a good home! I do really love the colours and tail shapes, I don't think they're rare, but they are beautiful.
Aquarium Co Op has great series about breeding fish for profit, how to breed and the common problems, and supply and demand issues from the retailers point of view. It's very interesting even if you don't plan to breed fish to sell!