Be informed - Read all contracts with venues since many venues require vendors to accept liability for things like slip and fall accidents that may be out of your control.
Avoid trip and falls - This is the number one exposure for crafters at a craft fair or flea market. Be sure that your area is clear of any and all tripping hazards.
Bring help when needed - Hire a helper for craft fairs or flea markets that may have many attendees. If you are working the booth by yourself, this could lead to theft of your product when you are helping another customer.