How do you pay for purchases?
Each vendor has their own practice. I generally use cash at markets partly because I am not overly trusting of vendors at flea markets, probably unnecessarily. Some will be equipped to take credit cards. But cash is the norm at markets and vide greniers. I have always used cash at Vanves.
Cash is indeed best. The area is safe (more than the area around Puces de Saint Ouen), so there should be no concern about carrying more than 100€ on your person.