If you want to get a rough idea of the worth of a car for free, check on mobile.de
. Notice that there will be a difference in price between private sales and those which are for dealers/export only.
This will actually give you a better idea of your car's value than Schwacke. If you have e.g. an older but cute intact convertible or sports car, Schwacke will list that well under what you could get for it, whereas large limousines will be listed higher than what you'll actually get for it.