A flea market can make an interesting poetry topic idea because of the volume and diversity of the people and things there.
There is so much at a flea market: innumerable products, different stalls, hundreds or thousands of people, food vendors, a variety of smells, a variety of colors and different languages. A flea market can be an overwhelmingly busy place. There is movement and buying and selling and eating and drinking and talking and sounds and so many other things. There are countless stories that are present.
With all of the volume and diversity at a flea market, it can make a very interesting poetry topic idea.