I usually just give away my extra fish.
I just moved some fish around this weekend and gave about 40 juvie africans to someone in the local fish club. When he came over to pick them up, he brought me a bag of extra fish he had. We both won that way.
If nothing else, you make someone really happy by giving them fish that are just excess to you. Up here, there's no shops that I know of that will give you credit, so might as well give them to other hobbyists.
Try to find a local club. If you want some money for them, see if they have an auction. Most clubs do, although you may have to wait, and don't expect to get a lot of money. At most club auctions, it's a buyer's market.