I found counterfeit products not only at flea markets, or at private parties, but also in department stores, chain stores and specialized boutiques. You can find counterfeit products anywhere. Of course, they are often found in places where the sellers can quickly disappear, such as flea markets. Many sellers are also street vendors.
What concerns me about your question is the idea that you can trust the supplier. Perhaps the buyer trusts the supplier, but that does not mean that the supplier is trustworthy.