1917, especially in reference to the marché aux puces in Paris, so-called "because there are so many second-hand articles sold of all kinds that they are believed to gather fleas." [E.S. Dougherty, "In Europe," 1922].
A market, usually held outdoors, where used goods and antiques are sold. For example, We picked up half of our furniture at flea markets. The term is a direct translation of the French marché aux puces and presumably implies that some of the used clothes and furniture might be flea-infested.