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.