While all forms of entrepreneurship share common elements such as the need for capital investment, permitting, inventory, marketing, sales, accounting and legal services, small businesses spun from hobbies have more flexibility than most. If you are an active numismatic, you might have already traded coin for coin or coin for cash, and that first swap is the seed to a business. How far you take it depends on your ability to spend time and money to turn your passion into a business.
Begin with your own collection. If you have a well-established collection, you can start by offering some of what you have for sale and using the proceeds to procure more coins.
Use the contacts you've made as a collector to find others to buy from and sell to.
Insure it. As you increase your collection for your business, be certain that your wares are appropriately insured against loss and theft. Nothing cuts into profits as much as a zero return on your investment.
Create a business. To truly begin, you will need to come up with a company name. If you are starting small, you need not incorporate or spend lots of money on accountants and attorneys. Create a name and logo and print up business cards. Create a website with a link to a Paypal account. Whether you choose to have a physical location as well is up to you, but it is not necessary with such portable wares as coins. You can do it all online.
Build your library. If you expect to do well in your new business, you need to become an expert on all aspects of coin collecting. The U.S. Coin Digest, The Official Redbook: U.S. Coin Digest A Guide Book of United States Coins and The Official American Numismatic Association Grading Standards of United States Coins are a few books you should have in your collection. Also, stay current with what's most in demand by subscribing to coin magazines such as Numismatic News and Coin World.
Do your research. While throwing a few price tags on the coins in your collection might yield a small profit, unless you fully educate yourself in all aspect of numismatics you will not be successful. Learn what coins are most popular and what you can sell them for. Set realistic goals for your inventory and your sales.
Practice grading the coins you have and those you encounter for sale. If you cannot accurately assess value and condition, you can't be a coin dealer.
Advertise. Build a Web following through your own site and on coin-related sites and blogs, but also advertise in traditional coin and collector magazines. Attend coin shows. Make contacts. Stay abreast of upcoming coin shows and events through the American Numismatic Association and other coin collector organizations.
Build a following through business integrity. Be honest about what you have, its value and the price you're willing to sell it. Honesty yields repeat clients.