Go for clown loaches then tell us how you did it. jk same as above, ask what they take or take your chances with some very pretty fish. Also usually chain stores will not take your fish only local stores might. I have two stores around here that fly for my fish, but only after they got to know me better and realized that I pretend to know what I'm doing well.
The wholesale prices for fish are so low that there is really no way that a home breeder can compete on all the most commonly available fish. If you want to sell fish start by specializing in something like apistos or rainbows or killifish or? You will have a much better chance to sell them to other hobbyists then to the LFS. There are still mom and pop stores around that might buy your fish but they are few and far between these days.
I sold seahorses for 4 years and I was breeding them when they were a very HOT item on the SW scene. My adults were selling for $70 each but as more and more came on the market the prices dropped to below $50. At one point I finally did the math and factoring my time I was losing money. Basically I was making back about 50% of my outlay. I did it as long as it was fun and then quit two years ago.
Technically, you don't breed guppies, you just wait. I once had a stowaway female guppy that was already pregnant...so I didn't buy a single guppy, and ended up with a bunch of little irritating fish that hide real well.
RCS are real easy too. Start with a group of 20 or so and no fish in the tank, and you'll be crawling with them in no time.
Some fish are guppy baby compatible. Many more are not. Raise guppies for your self not as feeders. Fish stores might take them off your hands without charging you, but they can buy a box of 500 consistently sized fish for less than the shipping costs. So, why should they buy from you?
Wouldn't it be more fun to breed high quality show grade guppies you can sell for $25.00 a trio?