Every German city with some self-respect has its own Flohmarkt (flea market)! This is where locals gather to sell things they do not need anymore and buy things they like. If you give it some thought, this is nothing more than a way of recycling!
Every second Saturday the Frankfurter flea market takes place on the Schaumainkai (the promenade in Sachsenhausen). The location is very central and enjoys a nice view on the skyline at the other side of the river. Various stands are set early in the morning to sell a wide variety of used things, such as furniture, potteries, decorations, clothes and bicycles. Some of the stands are permanent, but is is quite common that locals attend the market randomly to sell things they do not need any more.
If the flea market does not take place on Schaumainkai you can find it in Ostend. I personally prefer this location, as it can be combined with a very nice walk in Sachsenhausen and early Saturday coffee in K’ties?! The biggest secret to discover all the small treasures is to go early in the morning; I know waking up early on a Saturday morning is not ideal, but it is totally worth it!