Kleinanzeigen.de is the defacto monopolist for thrifting in Germany. I use it a lot and nade mostly good expierences. Some scammers, but they're easy to spot. There are rating scores divided in categories which are super helpful. There free items there, I got some great stuff for free there and gave away stuff that wasn't great for me anymore.

Otherwise it is mostly local facebook, whatsapp or telegram groups. I use or used these for freecycling, thrifting and foodsharing with only good expierences.

Another thing that might be interesting: depending on your area there might be give boxes. Small stalls or even sheds where people can take or leave stuff freely.

Kleinanzeigen was formerly owned by Ebay. The difference is there is no auction function on Kleinanzeigen, just straight sell/buy/give. Kind of similar to Craigslist. It is moderated (ads need to be approved but there is some automation to this I believe). It is huge so it has lots of things that are free but afaik no special category for it. It is possible to filter and search though. It is completely free to use the site and app. It is mainly for private use but companies can register there as well. Kleinanzeigen has their own payment system which is not mandatory but if you use it (with increased safety - payout after receival) Kleinanzeigen takes a small fee plus a percentage of the price.

I have never used any other site.