If you do not have experience using a canopy at a flea market, you need to be aware that they can be dangerous. A gust of wind or even a wayward child can knock your canopy for a loop and cause injuries.
Always secure your canopy as soon as you get it set up. Don't wait until a “better time.” The better time is now.
Whatever you chose to secure your canopy make sure the method itself isn't dangerous or hazardous.