I have 3 varieties of fancy guppies that I breed and sell. Also platies, cory cats, sometimes angels and bumblebee cichlids. I have come close a few times to being able to propagate coral cuttings, but something always happens where I mess up and lose them before I get them sold. Saltwater macroalgae is also pretty easy to grow, always somebody looking to buy some of it.
A couple of times I have found somebody wanting to sell a tank setup with fish and everything, you can sometimes get good deals this way, however it's not always a variety of fish you are interested in keeping. I have been able to sell these fish to the LFS also (I usually keep them around for a while to be sure they are healthy since I value the good relationship I have with my LFS), which makes the price of the tanks really cheap!