I used to be the "aquatic specialist" at a Petco and can tell you that they will not buy, only "accept" for free, to be given away for free, fish from customers. If you have employees saying they will buy your fish, maybe they are going to pay personally for them, but I can't imagine the managers wouldn't allow the store's funds to purchase fish from a hobbyist, goes against all the protocols. They get fish dirt cheap from their supplier anyway (mostly SunPet in florida, on the east coast at least for freshwater, and the saltwater fish from sister company liveaquaria.)
On the topic of live foods, if you breed and raise themselves there is nothing wrong with it, as long as the fish have a solid staple diet and the live food is more of a treat. It can spoil some piscivorous fish to not want pellets anymore which can be an annoying habit to break. The main problem with live food is that the store bought options tend to be disease and parasite-ridden because they are kept in such poor conditions, packed in like sardines. Also the goldfish commonly available are very high in fat and are not a balanced diet; and lots of uninformed people think they should be the staple diet of fish like Oscars or arowana.