Best way to sell used car?

Would anybody know what the best way is to sell a used car in Belgium?

While we're still hoping to get a decent price on the car (Audi, 2005) we can't imagine the whole hassle of selling it ourselves plus we don't know the local procedures & requirements. We tried "My Way" in Kortenberg but they said they'd only take our car if we bought one of their second hand ones and we're not sure yet if that's what we want.

Is there a garage you can recommend that offers good prices for second hand cars without us having to buy anything back from them?