Years ago any pet shop would buy fish from customers but today some do and some don't.
I recently came across a chain store pet shop and they refuse to buy fish from customers, which I found odd. But their policy is they don't buy anything from customers because they could be diseased. When I pointed out the fish they were getting from the eastern states wholesalers were diseased, and they had diseased fishes in their shop tanks at that moment, I was asked to leave
Most other shops will take fish tho and they usually give about 1/2 retail price in store credit, and 1/3 store price in cash. Some shops pay less than this but most pay these amounts.
If you want to sell large numbers of fish they usually pay less again but they take more fish.
eg: You have 200 male guppies to sell, the shop might take them all but they will offer you about 1/4 retail price in cash. The shop takes a chance on buying so many fish and they might not be perfect, and you don't get as much money per fish, but you do sell more of them at the same time.
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On a side note, when rearing tropical fish, the best temperature is 28C (82F). Do daily 75% water changes to dilute the growth inhibiting hormones produced by the baby fish. Keep the lights on for 16 hours per day. Feed them at least 4 times per day and feed them so their stomachs are huge and they look pregnant. Newly hatched brineshrimp is the best foods for rearing fry once they are a few weeks old. Microworms can also be added.