It is essential to have your business legally prepared. How you can ask? You need to have a business license, a home profession license, if running your business from home, a city or county license, (totally depending on what State you live) additionally you need to register your business as fictitious if you are not using your name as a business.
For example, if John Doe has a business name as John Doe's Grooming Parlor, a fictitious name isn't necessary.When opening a business in your residence, additional information is necessary. You can view some advice by Tom Wyckoff online and get to know more about the legal services.
Some businesses require more than a city license to be fully licensed. You should always check with your local city & state office for detailed information.
While it is my understanding that there are many businesses who are working "under the table", so to speak, I highly advise against this practice. If you are just interested in selling a few things out of your home, don't worry about this section, as you should not have any form of tax legal responsibility for selling your own belongings.
However, if you start a new business to make some money, you need to look at the legal aspects of running a business. In the end, those who operate under the table will be caught.