Here is my advice on visiting a rural vide grenier in France, based on my own experiences!!!!
Have cash - preferably in small denomination notes. More convenient for the sellers if they don't have to give change.
Be an early bird - arrive early - catch those worms - find those bargains! They will set up from 5am onwards! Bring a flashlight or torch!
Be prepared to rummage - they may not bother to put everything out and display it nicely - look through boxes. Be thorough - look under the tables for those gems.
Ask questions - they may not have put everything out, and may have just what you want sitting in the back of their car!