Good advice from RSMRyan, it's going to vary depending on your location.
Are you dead set on opening an online store? Do you have the coral stock and growth to support that? If it were me, I'd start slow with low volume and work my way up. If you only have a single frag tank and sell a few frags per month on Craigslist and on local reef forums, you're unlikely to attract much attention. Plus, by starting slow, you have low overhead and can grow organically, which will probably give you a better chance of long term success.
Once you start to move some serious volume, it would definitely be wise to make sure you're properly licensed and permitted. But, I think if you start out really small, you won't have any worries for a while.