Why? Among other reasons, it's because you will be attracting greater-than-normal traffic to your area. You'll need to check with your local city clerks about the regulations as they can vary by locale. Some cities and towns restrict the dates or frequency that you can hold a sale.
Additionally, each city or town has its own regulations about how and where you can post flyers advertising the event and what happens in case of rain. Others insist that the items you sell be your personal property and not acquired by you for the express purpose of resale.
This raises the question: How did you get enough sample items to sell? If you're doing this on behalf of an employer, you may be prohibited from holding the sale altogether.
Related: Who Says No One is Shopping?
Related: Two Weeks to Startup: Day 10. Hang Out Your Shingle and Sell