Everyone who has a successful start up business, big or small, did the same two things. They identified an opening in an available market, then they filled it. Before you spend a dime on equipment or livestock though, you will want to make a business plan if you are even thinking about turning your hobby into a business. Even if you are not planning to hit up your local bank for some venture capital (very difficult to find in this industry without a very good track record in the field), the business plan will help you organize your goals as well as your thoughts.
Just in case you haven't noticed, and you probably have, there is a huge move on in the United States and Canada, as well as other countries around the world, to ban the export and import of wild caught fish, corals and live rock. At the same time the market for these products is rapidly expanding. With the expansion of the available science, techniques and products, larger numbers of people are getting into reef aquariums because the odds of having a successful marine reef tank are very good.
How many times have you been into an LFS that either had no quality live rocks or corals because "they are too hard to keep" or had only a few because, "I can't find a good supplier." There is your open market. The LFS that thought they were too hard to keep can be educated. The LFS that couldn't find a good supplier is a potential customer just screaming for a quality product.
While you are at this LFS take a close look at what they have in the tanks. Ask the LFS owner(s) what they are looking for the most - decorator live rock, corals, etc. Try to get an idea of the price of the products in their tanks. Ask them how much they are willing to pay for the product. If it is half of what they are asking for the product, don't be surprised or offended. They are in business to make money just like you. They have the overhead of the store, paying the hired help, the electricity bills, taxes, saving college funds for the kids, and other expenditures to take into consideration.