I am a land development guy that specializes in zoning and land entitlements, the biggest thing about operating out of your home (or garage) is to understand what is allowed in the zone district (zoning) for your property. Also understand the details in your local municipality about the rules and criteria of permissible home occupations.
Lastly is understanding building codes. Your garage may technically not be suited for a M (mercantile) occupancy associated with a storefront with customers, but may be suitable for warehouse (ie internet, phone, or delivery sales). Understanding and defining your use within the confines of the applicable code is the key.