The only way you'll sell them, I think, is to find a pet store (not a chain, they won't take them) that is willing to buy. It depends on the store but guppies, platies and mollies will probably only get you around 50 cents each. That's a guess though, the only way to know for sure is to talk to the store owner.
Angelfish could get you more, maybe a dollar.
There are a couple different species of dwarf cichlid, you'll need to find out which one you have in order to sell them. Post some pictures of them if you can. Many of the more common dwarves that I see in stores are sold for around US$12, so I'm guessing you could get five dollars for a healthy one. If you have a more uncommon cichlid, like a shelldweller, you could make more.
You shouldn't need a license, but some areas have weird restrictions so you might want to look that up. Good luck.