The best bet is if you're lucky enough to have an LFS that accepts hobbyist-sourced fish for either credit or cash.
Find out what species they'd be interested in getting from you, what size they'd need and the ball park that they'd give you.
Unfortunately LFS and especially hobbyist-friendly LFS have largely gone the way of the Dodo, at least in my area.
Otherwise, I'd suggest just keeping and breeding fish for the fun of it and focus on reducing your costs. Join a fish club and you can find new species and equipment for much less than retail..or better yet trade. Grow your own food (I culture red wigglers) and buy as part of club group buys.
I no longer sell fish. Primarily I donate to clubs that I support like CCA, CCY, PVAS and ACA and give fish to people who I know that will take care of them. I've had way too many bad experiences and wasted way too much time dealing with selling fish for a few bucks!
Matt