Flohmarkt Hanse-Center in Bentwisch

In the organizer's words:

Several times a year, a large flea market is held in front of the Hanse Center in Rostock-Bentwisch. Here you can buy all kinds of great and interesting things between 10:00 and 17:00. It's best to be there early in the morning so that you can secure the best bargains.

If you want to sell your junk yourself: You can simply drop by at 06:00 in the morning without registering with your stuff and set up your stall.