A flea market was an establishment that sold second hand merchandise.
When Benjamin Sisko saw the Promenade on Deep Space 9 for the first time in 2369, he thought of it as a combination flea market and free port. (DS9 novelization: Emissary) Miles O'Brien described DS9 to Tosk as something of a flea market for the sector. (DS9 episode: "Captive Pursuit")
Sisko thought of Ornithar as a galactic flea market dealer when he met him in 2371. (DS9 novelization: The Search)