The majority of pet/aquatic stores don't take homebred animals, they will already have contracts with wholesalers so they reserve the right to refuse homebred fish. Especially common livebearers. The only time they may make exceptions is if its something particularly rare (of which platys, bettas and guppies are not).
You can try and sell them through classifieds or fishkeeping forums. But livebearers like guppies are tricky to offload because anyone can breed them, someone might take them on as feeders though.