I usually find asking prices for old games on Ebay Kleinanzeigen always to be somewhat higher than whan those things tend to end up on regular Ebay. Then again, I don't like barganing that much on Ebay Kleinanzeigen.
Where I like to buy games when I'm not looking for something specific is on the fleemarket. Lots of people just sell things for whatever they saw them going for on Ebay, but not always, and even if atleast you can actually look at the items yourself.
You also need to look out on who's a private seller and who's a commercial seller. The latter doesn't necessarily has to have terrible prices (especially if you buy multiple items they often give you a lower price), but some also just have eggregious prices (especially the ones selling Nintendo stuff!).
In my experience when buying from private sellers who don't know the market that well prices tend to end up a bit squished, they tend to overvalue a lot of their stuff while undervaluing what actually is worth something.