It depends on where you live, so research is important before you decide to start keeping fish you catch yourself.
Here in PA, you can take lots of different fish home for your aquarium - minnows, darters, suckers, sunfish, catfish etc. Sunfish have no size limit, and can be considered bait fish. The only stipulation about taking sunfish is you cannot use a net, or minnow trap. They must be taken by hook and line.
OP - I'm not sure a pet store will take it. In PA, selling wild caught native fish without a permit is illegal and I doubt most pet store bother with such permits due to the low demand for native fish.