About the only time I felt trading was a reasonably comparable choice to selling privately is when I looked at all the "cars for sale" and blue book options, and the private party options were roughly $2000 more than the trade values, but, since it was a relatively late model sports luxury sedan... it was still worth $20,000, and in my state, trading in a car at the same place you are buying gives you a tax break (6% sales tax) on the value of the trade, i.e. $1800 in this case. In this case, the $200 difference wasn't worth my trouble. But you probably shouldn't be buying from a dealer anyway, and I'm guessing from the URLs you posted, you are in Germany, so taxes and dealerships may vary from what we experience here.
My suggestion:
* Start by attempting to sell it yourself (on the sites you've listed) without investing too much on fees (but clean it up nicely and take excellent photographs
* If selling it takes too long or it seems the value isn't there, then consider seeing what you can get from dealers