Schwacke and DAT provide you with Dealer purchase prices, which are not necessarilly related to street re-sale prices and don't take into account regional variations. If you are in the market for a used car, better to survey the online sites such as Autoscout24
where you can get better feel of end-user pricing. Clearly some offers will be over-priced as people try to make more money on a sale, some under-priced for a quick sale, but for a common vehicle you can get a good feel for price ranges
in your region, variations by model, age and condition, and a mix of dealer prices too.