This is a collection of a few places that might make your life easier in Germany.
Kleinanzeigen (formerly known as Ebay, Kleinanzeigen)
Kleinanzeigen (“small advertisements”) is a platform where people can sell or give away things that they don’t want anymore.
You can find anything ranging from clothes, electronics, plants, art, furniture and even jobs (long term or one time).
As a German, I can say that all Germans use Kleinanzeigen as their main trading platform.
Not only is there a lot to find, but you can also put search requests where other people can react to (e.g. looking for a flower pot, for a student job etc.)
The only disadvantage is, that most people don’t speak english.
If you are on the platform you might stumble across:
VB: Verhandelbar (the price is negotiable)
15€ VB: Verhandlungsbasis (the price, being 15€, is negotiable)
15€ FP (Festpreis) (or nothing): The price is not negotiable
Zu Verschenken: To give away, for free
Versand möglich: shipping possible
