Flea market merchants are often transient and can be difficult to locate if there is a problem with charges. It's especially true for vendors who don't have online credit card terminals and instead make carbon copies of your credit card.
That doesn't mean those vendors are necessarily fraudulent, but it makes the transaction less secure. The credit card company might have trouble doing a charge back. If you're going to the flea market, take cash. It's also easier to negotiate that way.