This is a mix up , what you are allowed to do is sell you own used goods so a Vide Grenier situation but not more than twice a year most organisers of vides get you to sign a declaration that you haven't participated in more than 2 per year.
If you want to sell on a normal day market you will have to show your papers. Night markets usualy have a dossier of inscription most of which require your Kibis or your extrait de registre des metiers. Some very small markets , noel etc, do seem to waive the dossier for people they know and want at their expo/ market. I don't know how that would stack up if the inspectors came calling tho' and they seem to be doing that more frequently of late. Even at the Maison ede Gascogne marché de Noel in Auch I was inspected by the fraudes and the URSSAF last December.
I am sure this is a side rumour based on the vide grenier rule. Saying that you could sell your products at a vide twice a year if you think your product suitable . mine doesn't go too well at most vides as I make reversible children's clothes, so too many second hand cheap clothes for sale to interest people in my prices.
hope that helps clear up the mystery.